IMO, i would spend $90 and get a new gas tank instead of swapping in the old one. A used tank will still be full of crud, and have a limited lifespan. Autozone is about the cheapest place to get a new tank....i helped a friend put on in his '76 C10, and it was about a 2hr job. Just remove the straps, and cut the line that runs from the fuel sending unit to the gas guage (i think it is sodered on the sending unit). You may want to replace the straps too as long as you are under there. Also, the factory used tar paper as a buffer between the gas tank and the gas tank strap, and it would be a good idea to replace that as long as you are down there (any roofing tar paper will work, or rubber strips proably would work too).
The Carter AFB is a dead simple carb.....very easy to rebuild. The Qjet is a excellent carb when working properly, but it is pretty complicated on the inside. I would rebuild the Carter, it should be very easy for a first-time carb rebuilder to rebuild it. The Q-jet isn't impossible, just more time consuming.
On the HEI, it would be a good idea to replace the cap and rotor. Also, the HEI uses different plug wires than points, so they will need to be replaced too. You might as well put in new spark plugs, and gap them at .045" (the gap ideal for points is .035", but HEI will tolerate more of a plug gap)
I would also suggest getting a Crane adjustable vaccum advance kit, it costs about $23 and comes with the adjustable vaccum advance, and new mechanical advance springs. Then you can fine-tune your HEI to your engine. Also, HEI's usually like 10-12 degrees of base timing, whereas points is usually around 4 degrees. You need to advance the timing to 10-12 to fully take advantage of the HEI.
Good luck
