I have an 1988 Regal Velocity with 454 Magnum.
The exhaust exits the boat on the left and right rear sides of the boat. Also have huge fiberglass muffler.
What kind of stainless steel exhaust manifolds and pipes will work with this set up. And would I need a switchable muffler.

Please help?

LA

On my 1989 I had lightning headers that connected to a stainless elbow by rubber exhaust hose. Completely removed the mufflers. Sounded loud but good. Picked up about 300 rpms.

I had a 1988 23ft model, 454Mag, put a crane cam with I think 226/234 duration & had some head work done. I then added the Stainless Marine Hi-Torque manifolds and stainless risers. It was a fun boat & fast! I used the existing fiberglass elbows, I think I may have had to modify them (it's been a while), and then I took 1/2'' drill bit with an extension and drilled several holes in the baffillls in the mufflers. It had a nice sound but wasn't too loud!
If I were you, I would go with the Lightning headers with the reduced water flow and install overboard dumps for most of the water. The fiberglass elbows should still be OK, or you could opt for some stainless elbows! You should be able to do the same thing with the mufflers and it shouldn't be too loud!