The place I used to work charges $130 to hot tank/ caustic soda/ 205degrees (kill old stinky fuel and removed under coat and paint ) and acid tanks/industrial grade phosphoric acid ( neutralizes hot tank chem and kills rust), then wash the parts with rust inhibitor(sort term purposes) to keep it from flash rusting prior to paint( inside is usually if you don't use shitty fuel or let it sit forever). If your tank has any rust holes you'll know for sure but now it's perfectly clean steel and can be welded easily. I did this to all 3 of my Datsun tanks and had no issues and didn't use a liner.
There is a Datsun racing speed shop down in SoCal, I forget the name, and a while back someone here posted pics of the back lot and they have a few parts NL's. I called and he said he wants to sell them hole. Somebody might fill in the blanks.