Trucks like yours, prior to a 55.2, don't swallow a V8 easily. They tend to burp and fart a bit, needing to move the steering box over in order to clear the left exhaust manifold specifically. I would do all I could to keep an inline 6 in it. You might want to look into a 4.2 from a Trailblazer hooked to a modern overdrive automatic. I beleive that to be the ideal update for these old trucks that were designed for inline 6 cylinder.
Devin
