If the distributor is in the front of the engine and leaning toward the passenger side, that proves it's a big block. If not, it's a smallblock. If it's a big block, get underneath the truck and get the long number off the block. It'll probably have a dash and a single digit at the end of it. While you're at it, you might also want to get the casting date off the block. I THINK it's on the passenger side, near the long number. In any case, post the long (casting) number here and someone will tell you what it is.
That's true, it's just that it's almost impossible to see if there's an a/c compressor on the engine. The lowdecks are easier because the pad is in a different location. Or maybe I have which is easier backwards, I can't remember for sure. I'll have to go check one of my blocks to see.