Have personally run pumps with the extra nipple blocked off with no problems. Also seen many cars in the shop back in the day with the same setup with no problems.
Yeah, it's originally a non-return system, so the gas cap is correct.
I couldn't find a decent non-return line pump, so I picked up a Delphi unit (all others were Spectra or some other off brand) Ames has them as well as the other sites, but I was on RockAuto at the time and just grabbed it.
The one on the car now is about 12 years old, but not many miles on it and was a Carter unit. Works fine, but was doing a timing cover reseal and decided to replace it while I was in there.
Just so everyone is clear, all 67 and 68 birds require a vented cap regardless of a return line. only early 69's with the vent tube in the filler neck use a non vented cap and mid year they did away with it and went back to a vented cap.