Here are both, the aluminum flange and the sealing gasket:



You can see the tight fit onto the 4" tailpipe. actual gasket ID hole is 3.800, pipe OD is 3.995, this makes for a tight leak proof seal:



Assembled, gasket and the flange:




Now since this portion is done, flanges waiting to go to powder coating, now I can now get into making the transom hole oblong in order to get the proper angle to the riser. Right now the tailpipe is about 1 1/4" short of being inline with the riser so I've got to make a jig and re-bore the hole at the proper angle to get the alignment I need for the tailpipe to riser.

Just when I thought I was getting somewhere..................

This is a view looking from the transom end of the tailpipe up the tailpipe to the riser. Because of the swim platform the
tailpipe transom hole couldn't be raised enough to eliminate the mismatched parts. The silicone "HUMP" hose will take care of
any misalignments.



This is just a diagram (side view) to better explain the issue for those who may do this mod.

..... or noting your formidable fabricating skills, perhaps machine a thin "wedge spacer" for insertion between the riser and manifold to angle the riser/tailpipe further down.

Glad someone else thinks like me, yes, that did cross my mind......BUT......after considering the $$ and labor of purchasing material and machining 2 wedges and the extra gaskets needed, I thought of plan "B"......................

If you recall, I did cut off the flared end from the OEM Riser, that alone would reduce the close fit, but I wanted to have the exhaust gases shoot straight down the tailpipe eliminating a potential hot spot due to the mismatch angles. I have some 304 Stainless Tubing (2 3/4" ID) from an old exhaust project from a car of mine long gone. I'm going to angle cut the face of the 2 3/4" tubing on the front of the riser to match the angle of the tailpipe, weld on the 2 3/4" tubing to redirect exhaust flow to the center of the tailpipe. This will be far less expense and trouble then buying additional gaskets, stainless plate and machining 2 wedge plates. Visually when the "HUMP" connector is installed you won't even notice the variation angle between the riser and tailpipe. What'cha think ????

I really don't think the 2.75" extension is needed. With the fully water jacketed tails there should be no hot spot. Either way you are not out anything adding the extension.

Thanks for your feedback Ryan, I'll still proceed with adding the 2.75" extensions just because I have them and it makes for a nice install and just because I've nothing better to do with them......

Current status, tail pipe is done. Mock up stage before final weld on Riser elbow;

Are you running the silicone couplers dry. Won't they melt without water running through that area

Ooooooops, forgot to mention that tid-bit. Extending out from the riser you'll notice that 2 3/4" tube. That tube will be wrapped with Titanium exhaust wrap or Header Wrap as its also known so the silicone coupler tube will not see high exhaust temps because once the wrap is on and the tail pipe is pushed into place, the exhaust wrap will seal and exhaust gases from contacting the coupler, coupler will only see residual temps <400 deg. The header wrap will be securely wired in place w/stainless .045 dia. lock wire using multiple wraps.

I would look into 2 welded flats and a V clamp. Especially with the welding skills you have