I recently had dove into Pontiac's (hardcore Chevrolet guy) and bounced around with some different vendors. I have had the best results with Ames so far.
I agree with you and some of the others about maintaining the original tank. But that having been said, you could be fighting an uphill battle with debris and driveability. If I were walking your shoes, I'd but a new tank and sender, replace all of the rubber components, and call it good. I don't think you would have to look back.
As far as the undercoating, I'll throw my .02 in. Not only on the 76 Trans Am I have been working on, but the 80 Z28 I have as well.............I removed the undercoating to reveal a beautiful underbody. It is some dirty BS work, but the value it will bring it certainly worth the effort. There are plenty of people on this site that have spent the time to do the same thing and as a result, there is tons of great info and advice to be had.
If you were to look up Norcam & NJSteve's threads, you'll see great advice for products and procedures to deal with the undercoating, to say nothing for surface rusted components.
One my favorite threads is this...........................
https://www.yenko.net/forum/showthread.php?t=79126
Of course, the is Greame's (Norcam) thread...........
https://www.yenko.net/forum/showthread.php?t=157610
This is a great one for dissolving rust................
https://www.yenko.net/forum/showthread.php?t=155303
Hope this helps,
Tim