Just wondering if I need a vented or non-vented cap for my 68, behind the seat gas tank?
Are there differences in the filler neck as well?

 

It's a vented cap. Beginning in 1970, the cap could be vented or non vented---if it has Evaporative Emission.

B6TZ-9030-A (replaced 8A-9030-A) .. Fuel Cap-Vented Anti-Surge Non Locking (Motorcraft FC-23)

1951/69 all trucks / 1970/72 F250/350 / 1970/72 F100 without Evaporative Emission

 

Yes, there was a difference on the filler necks. The non-vented caps had deeper tangs on them to reach down to the deeper evap emission's filler neck. Ford did it so people couldn't get them mixed up.

 

Evaporative Emission filler necks also had a restrictor inside to prevent leaded gas station nozzles from fitting in. Back then, there were two sizes of nozzles, the leaded fuel nozzle was larger in diameter.

 

I don't remember that Bill. The way I remember it, 1975ish when the catalytic convertors came out they needed Unleaded fuel. The unleaded fuel caused the restrictor in the filler necks not evap. emissions.

 

I still have an adapter to go from the large nozzle to the small filler. My 86 and 87 F250s have the large diameter filler neck (8600 GVW) with the dual air pumps they were ok to use leaded fuel (if you could find it...)