Sideswipe served Centurion's purpose back in the day to allow the exhaust fumes to switch to the side opposite the surfer, but the valves to switch sides didn't last very long and were super expensive to replace. IIRC they were like $400/side when I priced them out. I decided to make my own version of the FAE conversion except with a single rear exit rather than dual rear exits, much like most boats that are coming out now with a single exhaust exit. This new exhaust exit was a 4" hole that i was not looking forward to cutting in my hull, nor was i initially excited about having two 4" sideswipe holes covered by a couple faceplates and 3M 4200. I've gone this whole season, probably over 100 hours so far with no issues though. The sideswipe cover plates were sourced from FAE for $60 and they included the shims for free--they are just a hard-wood shim though so any similar ones will work. You will also need a spray can of Debond (which works amazing!).
First up, rip out your old Sideswipe hoses and valves that go from the muffler to the side exhaust exits:
Once those are out on both sides, loosen the hose clamps holding the muffler onto the hoses that come off the exhaust risers and pull that out. Mine were on there good so I ended up using a combination of dish soap, wiggling and finally just kicking the end of the muffler a little bit at a time on each side to get it to slide off the riser hoses.
So much room now! Now you can fill the lockers with a big old surf sack without the exhaust in the way.
Next I moved to the Sideswipe exits. For this you need a 12-oz spray can of Debond (Walmart, $25 shipped) a rubber hammer and some wood shims. Spray the Debond around the edges where the exhaust meets the hull and you'll hear it start to make some light cracking noises within minutes as the Debond works on the adhesive. After ~5 minutes or so, work one of the shims between the edge of the exhaust and the hull, and it will begin separating. Continue this around the exhaust port, also tapping on the shims you've inserted previously, and the port will work itself away from the hull. Once it's completely separated you can use the rubber hammer to tap it out from the inside.
Now you can clean the area up with acetone, dry it well and then add the cover plates after applying 3M 4200 to them. Repeat for both sides. Add a bead on the inside edge of the plate where it will be close to the old exhaust hole, then another bead closer to the outside of the plate and around the bolt holes so you can sleep at night with the two layers.
Next you will need a 4" hole saw for the fun part, as well as all the parts to convert the exhaust to go out of the back. First though you need to get your new exhaust elbows, hoses, clamps and an exhaust tee, also commonly called an exhaust merge. For this I went with stainless 4" to 3.5" reducers right off the hoses coming off the exhaust risers on each side, then into 3.5" rubber elbows. Inspiration for this was from different projects on here but especially @Rugger's "Stealth Surf Pipe" thread. Reference that if you want this to look like a really clean install.
4" bi-metal hole saw from Home Depot - $30
1x Eddie Marine 100-31 3.5" x 9.5" polished stainless exhaust tip with flapper - $127.23 shipped. You can save $20 by not getting a flapper, but not worth it IMO.
2x Trident 4" 90-degree exhaust elbows - $50
1x Trident 3.5" 90-degree exhaust elbow, part # TRI-TRL-3590 - $20. Prices vary greatly, so shop around.
1x Trident 3.5" 45-degree elbow - $30
3.5" fiberglass exhaust tee for a MC 220CSX - $65. Any 3.5" tee with similar measurements should work, but no bigger. I got mine from Great Lake Skipper, but there were other (better) ones available but they were more expensive. I had to cut down each side maybe an inch to get everything to fit back there. The tee I used had a 24-degree angle on each side going into the tee, but I think a straight one could work better. Also, more of a Y than a T would likely work better as well. Still, test fit everything as you go as each application might have fit differences.
2x 4" to 3.5" stainless reducers - $40 from ebay
2' of stainless 3.5" pipe to connect the tee to the exhaust exit (both attach via elbows above). This will be cut to the size you need--alternately you can use fiberglass here as well. I probably used 1' here.
2' Vetus rubber corrugated flexible exhaust hose here. This was the most flexible hose that I could find, and I needed that because of the 24-degree tee that I used. You may be able to get away with regular 3.5" hose. $47 for this.
8x t-bolt exhaust clamps - $70. These are so much better than the stock clamps, but of course you can reuse the stock ones or save $ by buying new stock-style clamps. IMO, use the t-bolt ones below the waterline, and double up on each of those connections for peace of mind. I didn't have any issues with clamps loosening or leaking at all, but I checked them about 100x
Tube of 3M 4200 or 5200 (permanent). I used 4200.
I drilled my new exhaust exit after getting a pretty good idea of where the exhaust hoses would end up with the parts I had purchased, but before actually mounting all of them up. You can do it either way---pick an exhaust spot and then route everything to it, or get everything situated inside the boat first and then base your exit location on that. I went with this location after looking at a lot of pictures of newer boats with single exits, checking the inside of the boat for interference, all that stuff. Measure 100x and be sure your spot is good, because this would be a difficult hole to fit
I started by laying down painters tape on the outside of the hull to be able to mark where I wanted the hole to go, then drilling a pilot hole once I was certain my spot was good, and double-checked from the inside and out to make sure that was still going to work. This of course was after the 100x of measuring above. My pilot hole was an inch or so off from where I thought it would be (see the second pic), so I adjusted accordingly. Start with the hole saw in reverse until you are through the gelcoat, then have at it. It took a good couple minutes to get all the way through.
Now chamfer the edges to smooth everything out. This took a while because some of the inside layers of the hole cut tighter than the outside, so the test fit of the new exhaust exit kept getting stuck. Once it goes in smoothly but still tight (you want to have to tap it in with a rubber hammer so you know it fits without too much gap), mark the three screw holes that will hold the exhaust tip to the hull and drill those (reverse first, then all the way through). Be sure that the flapper is in the down position so that it works as intended. Finally, add 3M 4200 or 5200 to the edge that will seat against the hull as well as the screw holes and mount it in place permanently.
The order of inside exhaust connections I did was:
6" or so of either the stainless or fiberglass pipe goes into the exhaust riser on each side, and mates to the 4" 90-deg elbows. You want 3" or so of pipe in both sides of the hose so that you can get a solid clamp on both. Clamp securely in place on the riser side, but clamp the elbow lightly in place so you will still be able to move these around as you are test fitting everything.
4" to 3.5" stainless reducers off the elbows.
3.5" Vetus hose attaches to the reducer on each side, then mates to the fiberglass tee. This flexible hose was key to getting everything to fit properly.
From the fiberglass tee, add the 45-deg elbow, then fit the correct length 3.5" fiberglass or stainless pipe to meet the last connection, which is the 90-deg elbow attaching to the new exhaust exit. Here are pics for reference, but I wish I had taken more:
Now connect everything up from the inside and you're good to go. I finished this just before the boating season this year and never finished the downpipe installation outside the boat because I just always wanted to use the boat. I get no noticeable fumes with the exhaust exit in that location though, which is another reason I pushed off the downpipe install until this off-season.