73 and 74 Swinger door differences and question

You already figured it out but the side impact beam started in '73 with the body and interior redesign like a lot of stuff.

I would also suspect that the '74 mechanism is probably a better set up, seems to have fewer moving parts than the earlier ones. The only thing I've noticed about my '74 Duster's glass compared to my other cars is that it can be tilted more front to back, there's play in it in that plane which can effect sealing. But the mechanism seems smoother. The earlier stuff seems to hold the glass better front to back, but allows it to tip in and out. Who knows. A window hardware kit would probably fix most of that anyway. I would just use whatever set of doors has less bodywork to do on it and buy a hardware kit for the window and regulator.