It's been probably 15 years since I had one of these cars. My son was brought home from the hospital when he was born in a '71 250.
I am 99% sure that all it is, is a flare fitting, simply use an open end wrench, probably 3/4", and loosen the nuts on both ends and replace with the one way valve in the right direction. It sounds like you've already figured out the one way valve.
When you start on with the new one, thread both fittings in place with your thumb and forefinger, no wrenches. Then tighten both with a wrench AFTER you have gotten the threads started on both ends. This is good practice on anything with threads, don't use a wrench until it is started at least one full turn, this will keep you from crossthreading and ruining fittings, bolts etc.
Remember, "lefty loosey, righty tighty".
You can do it. I will be really surprised if you come up with any unforeseen problems with it.
Best of luck,
Last edited by LarryBible; 11-14-2001 at 10:43 AM.