GM V8 question

[spareparts]

generally, the difference is fairly simple, The marine engines are based on truck(heavy duty) engines with marine specific cam shafts and accessories. The exhaust system is water cooled and is marine specific. Usually, the bigger the engine, the better the mileage, the bigger engine works less to achieve the same speed as a smaller motor. Current marine engines have EFI available. there are a lot of differences iwith the programming in marine efi vs auto efi.

[smokeonthewater]

just forget efi conversion for mpg

It CAN be done but isn't worth the headache

if a car gets 20 mpg climbing a steep grade with a well tuned carb and 21 with efi then you have gained 5%

if a carbed boat gets 3 mpg and you gain 5% you end up getting 3.15 mpg

also where efi really shines is by leaning the mixture for low throttle highway cruising... boats NEVER do that because they are ALWAYS going uphill when planing.

[smokeonthewater]

also a 4.3 will bolt up to your bell housing but will require all of it's own marine manifolds dizzy etc and motor mounts will have to be bolted a little further back

If you can source the parts and do more work you can install just about any engine you want

as far as mpg tho the v8 will do nearly as well as anything else if you take it easy on the throttle.... you MIGHT be looking at as little as a 0.2 mpg difference vs even a 4 cyl

for fuel usage reference, many years ago I had a 20 arrowglass cuddy with a 305 (mercruiser 198 hp) and in a day of wot running and very aggressive tubing I could use up the 40 gallon tank of fuel twice.