1)Couldn't tell you any specifics, but I would look on this board or at one of the parts suppliers that sells body parts. I also have a 58 Fleetside, and I've been told the beds are hard to come by and rather expensive.
2)If you like tinkering, get the shop manual for the truck and you could even try rebuilding the transmission yourself.
If you want an original style truck, I would go for a three speed manual transmission with a column shift-3 on the tree-if you can.
3)Parts for a tune-up are available at the local auto parts store. The shop manual gives very good step by step directions on what to do. you may also want to consider a Pertronix Ignition system. They fit inside the original distributor, so you maintain the stock look, while eliminating the points and the need to adjust them. I installed one in mine and it works great.
4)For me the only problem this truck has ever had is the electrical. I am rewiring it right now. These old trucks didn't have the fused electrical systems that modern cars have. I believe there are only two fuses, both on the light switch. That being said, the electrical systems are much simpler and easier to work on than any other car I've had.
5) I'd leave the gas tank where it is. Yes it's inside the cab, but it is still sitting inside the rails and the truck is up high enough that it is unlikely another vehicle will strike it from the side above the rails, rupturing the tank. Even if that did happen, chances are, you'ld already be dead from the impact.
That's my 2 cents, and then some.
