should i use a vented or unvented gas cap. thanks

Since you ain't gettin any love on this Q I will start out with this: I would think with the tough fed gov't emission regs of that era (starting in '68 in most states, maybe 67 in California, not sure exactly) that it came with a non vented cap. gas fumes would be a major polluter. I think the C.A.P. (C.A.S)? system started in '68. I know my 68 440 GTX had the valve cover breather connected to the air cleaner as opposed to being open. Not sure how the tank is vented for the fluid to flow out/forward

Tank should have a vent that runs up into the inside of a frame rail.

The tank has 2 vents one that can be seen in the trunk area and routes to passenger frame rail. The other is on the left side and is J shaped and clips in by the upper shock mount drivers side.

The gas tank and cap in 1969 didn't care what engine was in front of it , no vented was the only cap whether a slant 6 or a hemi .

I don't even think there is a vented cap , if it was vented it would splash out fuel every time you hammered the throttle ....