Mercury ran the Alpha's on BBC's a number of years ago. On small blocks I've seen them handle up to 400 hp turing high rpms if you're easy on them.
One thing you can do that I think will be a good bang for your buck is to scrap the cleaver style props for a set of Mercury Mirage Plus.
You'll get on plane much easier and your mid-range cruise speed will be a lot higher. Top end speed will be about the same, possibly a little less but the boat will handle better in my opinion.
Good luck with her, she's a classic!