I had the same kunundrum with my 71. Luckily, dropping the tank is pretty easy to do, so you can just look inside with a flashlight to see what you're dealing with. For my time and money, it was 100% worth just buying a new tank, especially when you add up the costs of cleaning and coating the old tank, not to mention how big a pain in the butt it is to restore one. I posted a couple of blog entries with some photos of what I found in my tank, and the install of the new one if it's helpful:
http://www.1971chevysuburban.com/197...ank-sludge-yum
http://www.1971chevysuburban.com/197...s-tank-install
Good luck!