The gas tank and the stringers are the big thing in an X. The stringers have a notch cut in them for the motor mounts, and if not properly sealed are a direct path for water to enter and rot out the stringers. Check out the stringers around the motor mount, unless you are buying it as a project. A '76 should have an aluminum tank, which is better than steel, but I just had to replace the aluminum tank in my X-18 due to a crack. The X-18 is my favorite style Donzi. Where in MA are you located?
Post some pictures please.
1978 X-18, SBC, Volvo 280T
Big Grizzly 21 Solas Prop
Heart Bay, Lake George, NY