Hello all, I'm new here and I've come seeking knowledge!

I have a 1997 k1500 4x4 305 5 speed. When I bought it a few years ago, I did a few things and got it running tip top. I was getting 20 mpg, averaging ~19 mixed without being too careful and I'd gotten as much as 25 on a nice stretch of lazy hwy driving.

Well a few years, a few problems later, I'm getting between 16-17 mpg if I'm careful and I've been trying to get her back in shape. So far I've done my air filter (freshly cleaned k&n and maf) cap rotor plugs and wires (delco all save Borg Warner wires) fuel filter, oil change, and I've run seafoam both through the tank and the oil.

I'm about to do header back exhaust, drop the trans to do some work and do a fluid change. Figure full synthetic (as applicable) for the trans, tcase, and rear diff.

I'm wondering if I'm missing anything that could bring it back to what it was? I'm toying with the idea of swapping from my 3.73s down to the 3.42s among other things. Ideas?