Here's a strange one... My '74 'Cuda is originally an AC car. I'm going to put the dash (and heater box) back together with the original AC parts, but underhood I'm not installing or hooking any of the AC components up. I was wondering if there's any way to incorporate heater-only-car vents, for letting fresh air into the cabin. Any thoughts on this?

All the air doors in your AC/heater box will still work without having the AC lines and compressor up front hooked up. So yes you will easily be able to channel fresh air thru the box into the car, just like designed by Mopar.

Are you saying that there is supposed to be some kind of outside air vent on the AC cars similar to that in the non-AC cars?

Here's the thing, on the 'Cuda, I remember it getting very hot in the car while driving, and there was no way to enable outside air circulation, only what seemed to be recirculating inside air. (I should add, this was the case, even when I disconnected the heater hoses) In contrast, my '70 Swinger with no AC had little trap doors that you could open up, and cool outside air would flow in while you were driving, it would actually blow on you without the fan on, and with the heater control turned to off.

I think the non-AC E-bodies had a similar system, but it used two knobs that opened up some kind of flaps, that would accomplish the same thing as the A-body trap doors. With the AC, there never seemed to be any outside air flow, on or off.

Maybe I need to switch the under-dash heater box to a non-AC variety?

If its like a '70 there's a large fresh air vent from the cowl to the heater box above the glove compartment. If the box and vacuum pots are working as designed, setting the control on "AC" should get you fresh air through the AC vents.

"Max AC" will shut the vent and put it in recirculation mode - you don't want that.

It can all be bench tested by hooking up a mity-vac and working the control buttons. The doors should move well with 10" of vacuum.

I'd keep the original box and even get the evaporator pressure tested before re-assembling. Some day you may want to hook the AC back up. I like the factory AC setup better than the aftermarket variety because it keeps fresh air capability.