I don't think that boat deal looks too great. I think you could get a newer Scarab in the 30's that would be worth the difference. Mabye a 29 with small blocks and then you could keep the truck?
This is probably a better deal.
http://www.seriousoffshore.com/class...age=out&id=168
Oh, and for a 31 with twins I'd agree you'll want a 3/4 ton truck. Both for the safety/control and liability issues. Newer 1/2 tons can pull it, but controlling and stopping are the tough parts. You could bet the 3/4 ton gasser and save a ton of money if you don't tow a ton or drive a bunch of miles.