Cleaning a fuel tank

I’ve been slowly working on my ‘71 D100 and it had some gas in it that’s probably older than me. The inside of the tank looks pretty bad. The outside of the tank is perfect. I actually thought it was a new tank when I first saw it, but it’s dated July 1970. I’ve read multiple ways of cleaning out an old rusty fuel tank. What have y’all done? And how did it work?
I plan on eventually going to a rear mounted fuel cell, so I don’t want to spend a ton of money and effort on this tank. Just want to get it drivable for now.
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Thanks!