28' aluminum shrimp boat Repower

OK guys, posted on The Hull Truth but figured I might as well post here as well.

My grandpa has a 28' aluminum oil rig boat that he turned into a shrimp boat in the early/mid 90's.
Engine is currently a 3208 Cat from a dump truck and a Twin Disc MG 506 transmission, we have a Keel Kool system so the engine only sees fresh water, and we have a raw water inlet and pump for exhaust cooling and a raw water wash down.
engine horsepower is 150HP however after cooking the engine several times (figuring out the coolant system) we seem to have a warped head, and as the 3208 is not rebuildable we need to replace it, instead of replacing it with another 3208 we're looking at other options.

it looks like we will also have to replace the trans as well, these are the specs on the trans, the CS rep I talked to at Twin Disc said he thought a shrimp boat application would be medium duty, with possibly intermediate duty if we don't run it very often.
Max horsepower-

Continuous duty 126HP at 1800 RPM

Medium duty 150 Hp 2400 RPM

Intermediate duty 162 HP at 2100 RPM

Light duty 215HP at 2800 RPM

3000 Max RPM

Seeing as 150 HP is our "safe" max HP we would want to upgrade.

We're thinking 250-275 HP would be a nice start, the boat would (when working properly) hit 13 knots on the gulf with the winches installed, we're hoping to get 17 knots for a max speed and 14 for cruising speed.

We're considering a 6BT, we have not measured to see if the hull will allow for it, the 3208 is somewhat tight in the hull and I expect an inline 6 to be a good bit taller than the V8.

Do any of y'all have thoughts, comments, or recommendations?