What engine does the GMC have? I'm going to guess a 248 or 270, unless it's had an engine swap sometime in the past. GMC engine parts are a bit scarce and pricey, but with a little creative whittling by a good machinist, some non-original parts can be made to fit and work well. I'm in the process of doing some experimenting with using pistons, and possibly replaceable flanged cylinder sleeves for a Japanese Diesel forklift in 270 engines. Somewhere down the road for me there's a dirt track race car in the planning stages with a late 1930's Chevy body and a GMC 270 inline six.
Jerry
