If you are going to stay with a genuine GMC I agree with bigbadswingdaddy. If you go Chevy then a 292 with 9.0:1 propane pistons like mine. Long stroke inline sixes should have a race balance done to them reguardless which one you choose. Really makes then stay together longer and handle slightly higher rpms when needed. Some head work and maybe block decking. I had my head ported and polished for better breathability and more torque for climbing and pulling. Will soon have a modest 4 bbl and headers into the 3" exhaust I have.