Change the oil, check all fluids, new plugs, wires and Pertronix sells a upgraded HEI kit which comes with 50,000 volt Ignition Coil, cap, etc. Check and make sure the alternator is charging fine, check the battery, check the starter. Clean the carb if needed and run some B-12 or some kind of carb and engine cleaner through the gas. After that you should be fine. Oh and new air filter if needed. www.Chevyduty.com they have every panel you need to fix the rust. Get their free cataloge if you want. They have reproduction parts from overseas or something like doors, tail gates, fenders, etc that are MUCH cheaper than GM manufactured stuff.
Good luck and keep up posted. Get some pics if you can. I just got me a 85 Chevy (basically the same as yours since 73-87 are same). Good trucks, lots of parts for them, etc. Your luck to have the 350 already in yours instead of 305.