74 mach1 starting/charging and cooling issues

Your battery issue sounds like a ground issue somewhere - possibly your positive cable - but I'd start with ground. Pulling and replacing the fuel tank should have no bearing on the electrical system in our cars, so there shouldn't be a problem there. If you have a meter, maybe start with checking resistance of the 2 battery cables, ground cable to the block, ground to the body, etc. I think that's where I'd start, past that I think it'll probably be a crap shoot.

As far as your cooling issues - is your thermostat installed correctly? What temperature does it open - 185*, 195*, 165*? When you flushed the system, how did you do it? If you did it with just water pressure, did you run it through the system in the direction of flow or in the opposite direction? When flushing the system with just water pressure, it is more beneficial to run the water in the opposite direction of flow. They call it scale for that reason, it forms scales in the passageways. If you think about a fish's scales, they are relatively smooth from front to back, but if you run in the opposite direction, the scales generally pull up/out in much the same way a sail catches wind. With enough water pressure running in reverse, the scales should break free and rinse out of the passageways. Using a chemical flush of any sort should only help the "rinse" if you run it in this way. Other than that, what kind of air flow do you have through your radiator? Do you run an electric fan or is your car completely stock in that regard? If it is, do you have your fan shroud installed? How well is your engine running? Does it run lean? Is your timing good or is it too far advanced/retarded? All of these things are considerations to make with an engine that is overheating and shouldn't just be trusted to be right, check and double check to be sure and to rule that particular scenario out. It's not always the cooling system that is the culprit of an overheating engine. Case in point, my car was overheating in slow traffic. When I pulled my engine a couple years ago, I didn't remove the distributor at all for the work that I did. When I reinstalled it, I ended up checking the timing and found it to be timed directly at TDC. I had been running the engine that way for a few years trusting that it was good and blaming the electric fan for not pulling enough air through the radiator. I adjusted the timing and have not had one problem with overheating since. The lesson I learned is to not trust everything that you did prior to your current problem....

I hope this helps! Best of luck to you!!