Need help with exhaust design

Hi, I have a 17ft speed boat with a VP 270 outdrive, the engine is a straight 6 British Jaguar engine.
This Jaguar engine was used in small Tanks in the 70/80s and has masses of torque.
It came with a home made log type exhaust manifold.
It also came with 2 (3 ports on each) professional made ( Lancing Marine) aluminium jacketed manifolds.
These jacketed EMs have a water entry on the bottom and exit into the exhaust flow just by the exit hole, they realised this was a poor design,have upgraded te design with droppers, but are really hard to find let alone buy.
The homemade log type is fed into the bottom, does it’s job, and exits over the top and out with the exhaust.
I’ve re built this engine, spending money and time, so do not want water going into it.
The diy exhaust I don’t think I want to use at all.
The jacketed ones, my thoughts are, and stay with me here, is to seal the exit hole and create a fitting instead to exit the water. Fit a set of snails to each, fabricating an attachment piece to each, have the exiting water from manifold re enter the snail, flexi from snail to through hull exhaust tips with flappers fitted externally.
Each EM is 1 foot and 2 foot from the hull when in boat. 300mm and 600 mm
The oval end on EM will except a 3” hose

Any thoughts ideas, welcomed.