Some gas cap pics fror reference.

Oldmanmopar said

If you look at the pics 70 and earlier are nonvented. 71 and later are vented. I put a 73 tank in my 71 and I kept the 71 filler neck and cap on the car. What I did was drilled a hole in the filler tube. put a fitting in it and used a 1970 duster vent tube. On the other car I didn't put the vent tube in and it made no difference. So just use your 71 tube and cap. Steve

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Thanks for the info. I'm going to have to use a different filler tube as mine had some 'surface rust' from sitting around so long, so I soaked it in vinegar for awhile (too long, lol) and it now has some pinholes in it.
I'm also curious now as to how the early (pre '71) system vented since the tank didn't have the vent tubes like a '71 or the single vent tube (like '72-up) and the caps aren't vented, either.
I'm going the opposite direction as you did, I'm putting a '68 style tank that has no vent in mine.
I appreciate your input,
Dallas