A GMC 248 or 270 would be a lot easier to find and get parts for than a 302. Even civilian 302's are sometimes hard to stick in a small truck. Military 302's are a PITA. A 292 would be lots easier to find and build than a 302, if you don't mind sticking a Chevy motor in a GMC, and lots of folks would argue that it's a better motor.
The Stovebolt Forum has a great search function. I'm sure you could come up with some info on 228 tweaks. My hunch is that you could cheaply do pretty well just sticking with your 228.
I would also recommend joining the following forum which is dedicated to old gmc's. The guys there will be able to answer all your questions.
http://www.oldgmctrucks.com/page0026.html http://www.inliners.org/ is pretty good too.
