228 likely. Next time you look at it try to see the stamped serial number on the flat pad beside the distributor. The first digits are the displacement. And it will have GMC in an oval on the valve cover.
Unless it's Canadian...
Cruise on over to our friends at
http://www.oldgmctrucks.com/ and join their forum, post an intro, then post your questions.
Good luck!
Oh, if you buy it post some pictures of it. We love old truck pics.
