I can't Get A Locking Gas cap To Fit? 74' C10

[Cadillac Kid]

Hello,

I posted a thread awhile ago about not being able to find the correct locking gas cap for my 1974 C10 Fleetside Longbed. I'm pretty frustrated now about the gas cap. In my other thread I mentioned that I tried to modify the stock filler neck to accept the locking cap. (I know, this was the wrong thing to do). The cap I bought originally and still have is Stant 10572.

Today in the mail came another filler neck from a 1978 fleetside that I bought, because the stock filler neck is basically useless now. The locking cap fits in the new filler neck and stays in place if you twist it, like it should. But if you tilt the cap up and down and wiggle it, it just pops out. I feels like the width is too narrow and it is loose in the neck. So I have a new used filler neck, and still don't have a gas cap. Ive spent about 40 dollars in this issue and it is just stupid. Am I doing something wrong, or is the problem with these filler necks?? I know there are tons of people that use locking caps on these trucks, what is the secret?? If you have a locking cap on your truck and know the part number, or have a picture please let me know. I would really appreciate it!

Thank you

-Gavin