Sadly, after you go through all that trouble the rust will return in a few years.
I did that twice on my 68 GMC and finally had to replace the tank. Last year it developed microscopic pinholes on the bottom that were hard to see but sure could smell it.
If it's not too late here's some other tips:
Coat the rubber gasket with silicone or petroleum jelly to make it easier to slide the neck through.
Removing the seat is easier than to have to fight the tank through the back of it.
Tilt the tank up on the passenger side as you pull or if you have help, as they push on the neck.
If your seal is the original, order one from
Classic Heart Beat. (scroll down) It's a GM product and won't deteriorate in a couple of years on you. Then all you need to do is cut the old one out (makes it easier to remove the tank), slide the new one in, silicone it or lube it and slip the neck back in.