The banks turbo setup for the Mercruiser is perfect for that application, but would be almost imposible to adapt for automotive use.
The water jacketed turbo’s are expected to have access to the same cooling water, the marine exhaust manifolds are using.
I have watched a lot of guys build up turbo systems from scratch, and the most difficult issue seems to be making sure the correct fuel mixture is attained.
A blow through carb is often the easiest way, but not the best. Stock computers seem to have trouble with multiple bar MAP sensors. There are several good stand alone computers for controlling FI engines.
I noticed that Nelson Racing uses Electromotive Tec3 computers.
Turbo’s are supposed to be sized for your displacement. The only difference between twins or single, would be spooling time and plumbing.
My experience has been, the half second boog waiting for the turbo is quickly forgotten as you catch up to and pass, whoever got the jump on you. Lol