In 83 we ran a twin turbo set up on an offshore cat with great success. We had small bore big blocks because of our cubic inch limitation. Ran 7.5 compression, surface drives, put out 1003 h.p.at 12 lbs boost. Ran and won 11 off shore races without pulling the engines./ its all in the intercooling

Did you run water jacketed turbo manifolds? Or how did you handle the heat?

The turbos and the intercooler were all ram fed fresh water

All the high horsepower Merc offshore motors are turbo now. There are a few old school Gale Banks and Gentry kits floating around, or you might be able to source some water jacketed turbo housings. As said above, intercooling is the key to make these live. (and good fuel and spark control)