Can 68-70 B/E-body disk brake assemblies (less the spindles) be installed on an A-body if using 73-76 A-body spindles?

nope

No, you would have to use the B/E knuckles. Just bolt the whole assembly on there. Those brakes aren't as good as the '73+ brakes but they'll fit and work if that is all you have.

There is no 68-70 B/E system. There is a 67-69 B body spindle, and there is a 70-72 B/E spindle. 67-69 B was a bendix system with different caliper and mount and rotor. 70-72 B/E used the Kelsey Hayes system single piston caliper, bracket and different size rotor.

Didn't at least some '70 E bodies have a 2 piston caliper? Seems like I ran into that years ago.

No, all 70-74 E body used the single piston 2 pin caliper.... Dragon Slayer gave you the appropriate year applications above your last post...

FYI, the 73-4 E body rotors use the larger inner rear bearing and 1 pc rotors, as do all 73 up disc applications, the 70-72 E/B spindles/rotors use the smaller inner rear bearing and 2 pc rotor, hence 2 E body spindles, the 73 and up B body spindle is the "tall" spindle, similar to the F/M/J body slider caliper spindle....

Mike

Ok, so if I use the B-body assemblies including the B-body spindles, will there be any issues regarding the upper or lower control arms? These would be going on a 70 Dart which currently has sbp disks.

Yes there will be an issue with the upper control arm, the ball joint is too small on a '70.

Get yourself the 1973 V8 and up A body spindles, brakes and A arm and lower control arm, rotor with the 4.5 inch bolt pattern hub, and bolt it in and go stop better
I did that to my 1971 Duster street and strip, it stopped fine in the 1/4 mile running 134.+ MPH weighing 3450 Lbs. with me in that car
I used all the parts from a 1974 318 Duster with those stock disc brakes
I think the 1973 6 cylinder cars still had the small 4.0 inch bolt pattern them, Mopar drop them, the 4.0 bolt pattern, in 1974 is what i was told on here about them in 2002 or so when I built that car
I did mount the brake caliper so it was mounted behind the spindle by reversing the spindles from the right side to the left and vise a versus, it aligned fine and stop great with that change That help move some of the weight back towards the rear of the car helping traction without being seen

dr. diff has ball joint adapters that will allow the small ball joint to fit the B-body spindle.
i used these on my buddy's 72 duster and they worked great !

That is one solution.

If I use 73-up A-body upper control arms, will that do the trick?

Yes, that is another option.

i forgot to say, the reason i/my buddy went with the upper ball joint adapters, was the upper arms came with new bushings and ball joints already installed.
if your upper arms need any parts, just replace the arms with the newer style and be done.