I’m in the process of restoring my 70 cuda. With the 8 3/4 rear axle I found the wheel studs were all left handed. Is that correct? I thought only one side was left handed.

Then somewhere in it's lifetime, someone put a left drum, or rotor on the right side, of the car.
Then as far as rears go, in the axle shaft, the same thing, for whatever reason, Who knows

Left side gets left hand thread

My assumption..Somebody "cherry picked" the right side axle from your axle and put it in their own.
Then jammed their old lefty in the one you bought to make it a "complete" rear axle assembly.
Simple fix,though.
Order new right hand threaded wheel studs.
RAYBESTOS # 4435B
Regardless,i would check the axle bearings' condition no matter what.
Hope this helps

Yep, that helps a lot, kind of my thought too.

Are the the adjuster nut & associated parts still there or did they just swap in a LH side axle?

just swapped it in.