My son had an 80 step side a few years ago, when he got it it had a cracked tank. It wasn't leaking when he got it, but it had some kind of glue, putty,etc on it covering the leak. I don't know what the PO used on it. Or how long before my kid bought it. Whatever it was held good for a while. I don't remember if it started leaking again on it's own or if he picked at it.
Anyway he found a guy who has a plastic welder ( I do but don't have enough experience with it to have felt comfortable trying a repair like that) at a local body shop that was sure he could fix it, the kid took it out and to this guy 3-4 times ("he 'guaranteed' his fix would work and told my kid to bring it back if it leaked) a couple of times it went anywhere from a week to a month before it started leaking again, the other time it leaked as soon as he put gas in it the first time upon getting it back.
We hunted all over for a replacement. Most places poke gas tanks as soon as they get them in (talking junkyards there). There's one yard near me that has quite a few 60s-70s vehicles, (still there but no longer open, I've been talking to the neighbor and trying to get in there for a couple of years off and on) but they have some '74-75s that we looked at, way different especially in the location of the filler neck.
. Somewhere I found out '79-80 only. I found one out of an 84 ( that place said 81-87 all was the fitment) that looked like it "had to work" and I didn't get under to see but the kid tried and said no way. Eventually I found a hole in the wall junkyard that was hard to find because of all the overgrowth in front of it, and they had a 79 3/4 ton long bed. Remember my kid's was an 80 step side. That 79 tank went right in.
I found out later I'd have had to get the cross member out of the 84 that I first got for him to put in that truck and swap that out and that tank would have them went right in....
On another note I know for a fact that with early 90s Dakota's you can put the 22 gallon tank in place of a 15 easily
when I was messing with those. Short bed, long bed cab and 1/2 don't matter. Same straps, same fuel pump/sender and everything.