This is what it looks like with a stock locking cap, they are very rare nowadays and if you have one, the rubber is usually cracked and it'll leak. The lock is also easy to break/wear out on a stock one.
The 71 fuel tanks and up are designed to keep slight pressure in the tank and the fill cap should seal tight and not be vented...pressure-vacuum.

This is what it looks like with a aftermarket locking cap. It's what I use.

Here's part numbers for Car Quest locking caps. Napa or someone like that may be able to cross that number for you.

Lastly use this picture to help you get the right cap for your filler neck. The 35012 is on the left in this picture.