Don't fix what isn't broken, remove the sender while the tank is where it is and look inside, Mike never did anything to this tank as far as I can tell, it still had the rotten float inside it, if he had done anything with the tank I am sure he would have fixed the float, and this NL320 was pulled out of the bushes as I recall.
Once you dip that tank, if it has holes in it afterwards, good luck finding another one, they are not the same as the L320, they are different, and I don't have an extra for you, I cannot even find a good rear bumper for these NL320s, as everyone that finds one is trying to save it, they are not parting it out, you are lucky that both your taillight bezels are good, 320 parts are drying up, and what is out there people want a fortune for, someone recently wanted a $1000.00 for a set of NL320 taillight bezels on Facebook, don't know if they sold or not as I don't do Facebook.
I would not mess with the tank until you know it has rust in it, it is a very big pain to get in and out, and you will not be able to get parts/hoses for it, you will have to make everything including the gasket between the sender and the tank.
As for the brake parts such as brake cylinders and such, not sure if they are available either, as for the star wheel, hit it with PB blaster and use punches/sockets to tap it apart, once apart clean them up and put them back together, I do it all the time, but it helps for me to have had two other 320 axles to remove parts from in case I ruined what I had.
You need to save what you have, 520 and maybe 521 parts may work, but availability is always an issue.
You are not going to be able to afford all new stuff for one of these trucks, you will sink a large fortune in money and time making it dependable in your mind, just get it going and fix what needs fixed as it breaks, get the brakes done with new brake cylinders and get a new master, a 521 brake master will work if you use the 320 rod, then get the engine running using a gas can, once it is running blow air thru the line from the front to the tank and listen for air at the filler, then put a couple gallons in the tank, connect up the hoses and see if you can get it to run, use an aftermarket clear filter in the fuel line near the inner fender well and keep looking at how much crap is in it, if you see rust, then drop the tank, if you don't see anything but gas then call that part good for now, as far as I can tell that is what Mike did, and he daily drove it without a fuel gauge, he just kept track of his mileage.
If you were closer I am sure if it was going to run, we could get it running in a couple hours at the most, I have done it several times with engines that sat 10/20 years, I have this routine I go thru to start one, I have never failed yet except for one diesel engine that had been in a swamp for over 20 years, and it was drug into that swamp because it would not run, but I did get it to hit a couple times, but it never ran.