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.