What Mike said; I doubt you will get much more from your 350. Some other considerations: If your truck is a half ton, and you swap in a big block, then you're going to put the axles in jeopardy from the added torque. A 383 or 400 would probably tax them to the max, but a swap to one would use all of the same accessories and stuff you already have on your 350. On the other hand, if you have a 3/4 ton, a big block will work quite nicely, all you need is the right big block parts to put it in - the small block items won't work, for the most part. I had a '79 3/4 ton 4x4 pickup, and a '79 3/4 ton 4x4 Suburban that I used on rough country off road job sites and hauled job site boxes and a tenK trailer; they both had 400/TH400 that I worked over a bit with rv/torque cams, high rise intakes, and headers and dual exhaust. They worked real well, and I never broke anything. I also had an '88 4x4 dually that had a 454; it would probably have pulled both of the others together, with trailer and loads.