Back to the original question about the push in caps for in cab tanks. I used to use Stant caps on my truck because that's what my local parts house carried. I had to replace them every 6 months because the seal would crack and start leaking especially on right turns . I finally decided to try a different brand so I went to O'Reilly's and got a murray cap. That was 3 years ago and it still hasn't leaked.

 

Have some addtional feedback on the fuel cap soga thing.
One would think that a simple fuel cap is a no brainer, but not so.

As stated before, the STANT p/n 10810 listed on the STANT site for vintage applications 94 & older shows this cap will fit 77,78,& 79 style side Ford light duty trucks. This cap has a shallow depth cam. I have this cap ,which was purchased, new. This cap will NOT fit the fuel filler neck, because the cam is too shalllow.

Looking on the Autozone(AZ) online data base, it shows pn 6749 for 1977 & 1978.
The depiction of the cap shows greater cam depth than the STANT 10810.
The cap is designed with a vacuum release feature in case the carbon filter evaporative canister plugs up ,so the tank will not collapse. The cap is designed for a non pressurized system.
The AZ store had this cap on the shelf. The fender well cap fit perfectly to the fuel filler neck.

Still do not know if my filler neck is stock 1977/1978 or an after market, but suspect the fiiler neck was transfered with the bed swap on my 1973 F250.

The AZ online data base shows p/n 6810 for the 1979 style side, fender well.
This cap incorporates a shallow cam depth. The cap is designed for a pressurized sytem with a pressure relief valve in case the tank is damaged from collision impact.

Checked the data base on Gates. Gates shows p/n 31720, which also has a shallow cam depth and would not fit.

Checked the genuine auto parts(NAPA) online data base.
NAPA shows p/n BK 7031684, which again has a shallow cam and would not fit.

All I know, right now, is the AZ fuel cap 6749 for the 1977/1978 style side fender wall application fits like a glove and is not loose.

Believe the data bases of Gates, NAPA & STANT are incorrect for 1977 & 1978 fender wall applications. Believe these caps will fit the 1979, however.

AZ has it right.

 

99 = Styleside Pickup / ev/em = Evaporative Emission / FC is the Motorcraft part number prefix.

 

Notice for 73/79 F series that 79 is not listed for fender well caps (99) with the two bend up tabs, only 77 & 78.

Believe the 79 cap is different than 77 & 78 and is designed for pressurized tank system.

77 & 78 is designed for non pressurized tank system with vacuum relief feature in cap