Saw one of those for sale with a Merc Reman 300hp 350 hooked to a Bravo 3.
You could upgrade to the newer 4.5 V6 at 225hp; personally no one has ever complained of too much power. 300hp 350 at least. Beyond that you need to consider maybe changing the drive as it may not have a long life...
My '72 Road Runner that I put together on a shoestring budget and daily drove until the engine caught fire. I put it in storage, then went into the service shortly thereafter. I absolutely wanted to save it but had I brought it with me, hurricane katrina would've claimed it just like it did my...