First, is a locking vented gas cap available 4 a 1971 F100 ? I have not been able to find one in the past. There maybe one available now that I cannot find & I need to know quickly because I need a gas tank very quickly !!!
To make matters worse I plan to use the charcoal canister line as my return fuel line on a regulated system. Will this cause me to still need a vented cap ?

 

If you eliminated the charcoal canister system to use it's hose then yes you need a vented cap like would have come on a truck of your vintage without the system. Which I didn't realize still existed at that time, but apparently did on the heavier GVWR trucks all the way up to '77 in some states.

The problems may arise with your wanting a vented cap on a system designed for a non-vented cap with the charcoal canister system. In the Bronco world you could not interchange them because the locking tangs were different lengths, shapes or designs to stop people from doing just that. With Broncos, you can't put a '71 cap on a '69 filler neck and vice versa.
Probably the same for your truck. Might be why it's been hard to find one to fit?

The old-school solution was to simply drill a small hole in the top of the cap inside the gasket area. And then to NOT over-fill your tank like most of us used to do back in the day. In other words don't fill it up until you see it at the top of the neck.
If you never do that anyway, you should be free of most leaks. If the tank is that full on a cool morning, then by the time the outside temps have gone up 10 or 15 degrees, your cap might "vent" a little more than you like!

That's what I think I know anyway. I'm by no means an emissions, fuel, cap-modifying expert. But I know a lot of people have done just what was described.
I've never done that on an evaporative emissions equipped vehicle though, because I feel the fumes staying out of the atmosphere of my garage, driveway, shop and just the overall atmosphere in general is important.
I prefer to run a dedicated return line and keep the charcoal canister.

Good luck.

Paul

 

I have no way of proving it, but I think a Bumpside vented cap will fit on your non-vented fuel neck. I don't remember the depth of the tangs on pickups being different until 1977. The 77 and up gas cap tangs were vasting deeper on the ev-emis caps. Go to a local parts store and try fitting their 1968ish vented locking cap on your rig when you can.

 

I have been running these for years. They are vented.
Bob Drake Reproductions 01A-18416-A Bob Drake Locking Gas Caps | Summit Racing