Welcome to the Bolt'
A good upgrade would be the 302 GMC six, hard to find and pricey, but a great engine with power. The 270 is a good option. I am in the process of installing a 302 and a 4 speed overdrive, it will have a 1959 rear end with 3:90 gear ratio.
Just to let you know if you aren't aware, the Chevy 235 engines are shorter than the GMC'S.
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GMC Casting Numbers
