Like Coilover said, but here are some "typical" upgrades.

1.) Swap over to electronic ignition. You can pull a GM distributor out of the bone yard fairly cheap and this is probably the easiest way to get better performance out of the engine that is already in the truck.
2.) Adjust or rebuild your carb as it can drastically affect your mileage.
3.) Start cleaning up the "little things" on your truck that don't work. What you may find is that all of the "little things" add up to something big.
4.) Keep the 250 but ditch the old tranny. Pick up a T-5 out of a mid-late 80's S-10 pickup or find a donor 700R4 automatic. The overdrive will increase your highway speed without your motor sounding like it is about to blow up.
5.) Find a 70's c-10 donor for the P/S & P/B upgrade. I say 70's because there is an outside chance that you could find a truck with the same motor that you are running which already has the necessary accessories for you to strip and install on your truck. You can go all the way up to '87 I think for the disc brake setup.

If you are looking to really change your whole setup you can run anything in your truck up to a 400 small block without changing the engine mount pedestals. You would simply need new motor mounts and a few other modifications to make the V8 work in place of the L6.

Welcome to the 'bolt!

Daily Driver 63 C-10 LWB Fleetside