My 16 footer has a 90/65 on it and that was the factory build.
You said your boat is welded, so it should not be a problem at all....(holding the motor, not the HP regulations).
The 5.7L Hemi has MDS. Basically, it runs on 4 cylinders unless it needs all 8. The mileage is almost exactly the same as the 4.7L V8, so I went with the Hemi.
My last truck was the Land Rover parked behind the GC in the picture. It's a 3.9L V8...the GC gets better gas mileage, has waaaaay more...