What Fuel Cap Is Correct?

[Cadillac Kid]

Hello everyone.

I am going through the fuel system on my recently purchased 1974 C10 longed fleetside pickup. It has dual tanks. When I bought the truck it had two locking gas caps with no keys. I got the caps off. Im only getting the drivers side tank woking first. I preferred a locking gas cap so I ordered Stant (#10572).
It would not latch into the filler neck. I was upset after waiting for it in the mail and I tried modifying the filler neck to make it work, I know… So i probably have to get a new filler neck now. My question is, does someone have part numbers for the correct gas cap for this truck locking or non locking? When I ordered online there was many different sizes and lengths and models that all said the were compatible. So I'm out 10 bucks on this gas cap, I want to get the correct one next.

Thank you for all of the help.

Thought I would post a picture of my truck since I haven't yet.
The camper has been on the truck since day 1, so the paint on the bed is the best paint on the truck.
The other car is my daily driver, 1972 Cadillac Coupe Deville.