[Fr. Frank]

The gear ratio in the Alpha One drive you already have will probably not be the same as you'll need with a V8. This problem is easily solved by changing propellors. Unfortunately, you can't just "drop in" a V8. You will need to change everything from the gimbal housing upward. Same problem, by the way, for any engine that doesn't use that same Mercruiser big-block 4 cylinder.

That being said, that big-block four cylinder is available in factory versions up to 190 hp, and I have even see a Wellcraft Airslot 24' with twin "470" mercruisers that had been outfitted with Paxton superchargers. The owner had gotten them off a couple of factory supercharged AMC Gremlins. (No, I don't know why AMC offered a supercharged Gremlin} He claimed 220 hp from each engine, and said he had over 200 hrs since the install, running them "8% under-driven", whatever that means.

If cost is not a major issue, or at least not the overwhelming issue, I TOTALLY agree with Brian, go with the diesels.
Here's something people often overlook about diesels: At a given horsepower, a diesel will produce 20 - 50% more thrust/torque than an equal horsepower gasoline engine.

For instance (going aviation here) a Cessna 172 with a 180 hp six cylinder gasoline engine will have almost exactly the same performance characteristics as the same aircraft equipped with a new 120 hp four cylinder aviation diesel engine, and yet the diesel will use just about exactly half the fuel at cruising speeds, and about 15% less fuel at maximum speeds. That changeover to diesel power, while expensive, effectively increases the cruising range of that aircraft by more than 75%, from 720 nm at max weight, to over 1250 nm at max weight.The same principles apply for marine use.

The question you need to ask is, do you want to spend the up-front money for the repower with diesels.