I've got a 71 as well. Saw somewhere that our 71's have a one year only tank filler. Don't know if that is true or not. Anyway, what I ended up having to do on ours is just go down to Kragen/Schucks/Checker and trying the caps. I ended up finding one that fit pretty well. Only issue is that I doubt it will last too long. Once the rubber seal perishes then it will probably fit loose. Would still be on and stop the casual siphoner, but would probably be fumey. Thinking of swapping on a later model filler tube to get caps that fit right. I assume they will just swap without too much bother.
Cheers