There's tons of stuff you can do, but most of it won't pass a smog inspection if that's a requirement. Even if that isn't a concern it is labor intensive or the PCM can't adjust for the changes. You can get lots of power with a set of Hedman headers, but they are a pain to install because of the modifications you have to do, and the PCM doesn't adjsut well. You could also add a mild cam and port the head, but the PCM may or may not like the changes.

What year truck are we working on anyway? It's kind of hard to make recommendations when we don't know what we're dealing with. I've been assuming it's an 88-96 model. If it's carbureted there's a lot more you can do.