Your best option would be to find a 248 or 270 GMC engine and install it. The original engine in that truck was probably a 228, but "There's no substitute for cubic inches", and the bigger engines are virtually a bolt-in swap. The biggest GMC six, the 302, has a few adapting issues, IIRC. All the GMC's have full-pressure oiling and insert-type rod bearings, even the older 270's from WW II surplus 2 1/2 ton trucks. Those engines were very popular with dirt track racers in the early 1950's when my family was running flathead Ford V-8's in a 34 Ford 3-window coupe. A good-running 270 could just about keep up with the flatheads, but the 216 Chevy engines didn't stand a chance!
Jerry
