1976 Dart Parts into Early A?

Not in '76, it didn't. Due to tightened brake performance and pedal-effort standards that came into effect a couple of months into the '76 model year, disc brakes w/booster became standard.
Well, some must have slipped through as I've had two '76 A-bodies with manual drum brakes that looked original. I didn't note the production date but maybe they were real early ones. Of course, they could have been conversions of some sort but they had the '73-6 spindles, drums, steering, upper arms, etc. so it's unlikely. I know of several local guys that have bought '73-7 A-bodies to get the disc and 8 1/4 rears for their early A-bodies and once they swapped them over, they put the old drum parts back in the later A and sold the car so anything is possible. My point was, don't assume anything. Look closely before you buy so there are no surprises later.