Awesome on the dual tanks. sounds like your front tank sending unit is bad. not sure if you will have to replace the pump with the sending unit or not.. might be worthwhile to do so if you are going to pull the bed or drop the tank to do it anyways.
As a daily driver for a vehicle that old, especially driving it nearly 600 miles in a day I would be looking at a newer vehicle with better gas mileage. However, I totally understand the concept of "with what money, idiot?!" and if that is a part of the reason for driving a 27 year old truck that much as a daily driver, I would entertain getting a backup car for when you do have to put the truck down for maintenence (and it *will* happen...Just a matter of when). an 800 dollar beater will not be that much more to insure and keep running, and gives you a backup plan. (been there, done that, bought the t shirt)
but as the first reply to your original post says, tune up. As for brakes, I wouldn't mess with it. not worth the hassle, and you won't get that much better stopping power compared to the drums anyways. Plus you will very possibly put this truck out for repair/ maintenance for a few days to do all of this work..(see above comment on having an extra vehicle)
I will also add, spend some decent money on new good quality tires. that long of a trip and you are asking for trouble on old mismatched or worn/ cracked tires..
AJ