Need help to ID my 8 3/4 rear.

Technically a Duster is a Valiant. Plus I'll bet that Plymouth had a heavy duty rear end package for say a Valiant that was a government car. They didn't make 4dr Swingers so if you bought a V8 Valiant 4dr sedan I would bet you could get an 8 3/4, especially if it had a manual trans.

I would say that you have an A body rear end. The spring perches are 43", the leaf springs will add about 1.25" per side x 2 sides = 2.50".
43 + 2.5 = 45.5".

The backing plates are 57" wide on the inside.
57 - 45.5 = 11.5.

11.5 divided by 2 = 5.75" from outside of the spring to backing plate