There is a product it's called all-seal its in a yellow tube I use it for trail fixes if it is under pressure use a little aluminum duct tape. Works great I had to do it to my tranny pan and fuel pump!
Hey Man.. I had a 1974 J10 and the exact same thing happened to it. One day it was like.. oh crap the fuel tank is leaking everywhere!! It turned out that the rubber fuel lines had cracked from age dumping fuel all over the tank. It gives the impression that it's leaking everywhere, but really it's just spraying out of those cracks in the hose.
I'd replace those first before you get too carried away with replacing the tank. While you have the tank dropped you can also give it a good look over. New replacement tanks aren't that expensive if you find problems.
If the tank has a problem they are fairly easy to drop and you will then have proper access to assess the condition of the tank and all those old rubber parts. It may indeed just be the rubber bits, they were not designed for 30 years use.
Fuel tanks are not that expensive so if it was leaking you may consider it easier to just buy a new one than try to patch a leaking one. The fix to a rare or expensive tank is usually to coat it internally and there are plenty of products on the market which can do it.