"There's no replacement for displacement". All the GMC sixes are pretty much bolt-in swaps. Find a 248, 270, or 302 to swap for the 228. GMC engines are scarce and expensive to modify, since most of them were used, abused, and and destroyed back in the 1950's and 60's by dirt track racers, drag racers, and Bonneville land speed record runs. The days of having thousands of WW II military surplus 270's and a few military 302's from the 1950's are distant memories. There's a small group of hard core GMC hotrodders still out there, but it's a pretty exclusive bunch.
Chevy 235 and 261 sixes are much more common, less expensive, and they share the same bell housing bolt pattern and a lot of other bits and pieces with the GMC's. The "go fast" parts for Chevy engines are also a lot less pricey than the same stuff for GMC engines.
Jerry
