3.6 GT powerhead details

[Hillclimber]

Pleased to say the car had it first race last weekend, at least it finished. I now have a 30 point snagging list to complete over winter before next season. With the crankshaft horizontal the exhaust exits through the "Passenger area" with a rectangular pipe and silencer(muffler) I made including a copy of the megaphone. This has created too much back pressure at WOT and the smoke does not escape correctly. I am now going to fit the original inner exhaust housing plus original megaphone.
QUESTION: When a V8 is on a boat with exhaust under water, what is the pressure at the exhaust outlet?
I am considering fitting one big or two small Turbos to use their natural ability to reduce noise level. The generated air would be used to cool the inner exhaust, not (yet) to increase engine power.
Any help/advice appreciated. I have to get down to 105db noise level before I can race.