Welcome to ***boat!
Your on the rite track. Don't get too crazy with the cam or you wont get the idle down low enough to shift it without hurting the alpha. You will need to upgrade the exhaust system to center rise manifolds if you still have the stock type rear rise manifolds.
I have a 79 AMF Crestliner 22' deep v cuddy cabin. A trailer shop told my boat weighs close to 5500 lbs loaded. It started life as a mercruiser 260 alpha. It now is a .030 over so 355 cid with flat top pistons. I have 194 fulie heads on it and a maine/rv off the shelf hyd. cam (dont remember specs). Still running the stock cast iron intake and q-jet carb. I run OSCO center rise cast iron exhaust manifolds. The motor is balanced.
Running wide open and trimmed up I can GPS 55 mph. I usually cruise 35 -37 mph when the ocean is flat. Its just my fishing boat I don't like to beat on it so I don't open it up often, just when the ocean is flat and the fish ain't biting.
You didn't mention where you boat but if salt is not a concern running aluminum-heads, intake, and exhaust manifolds would save a bunch of weight and help you go faster.