B-body 8.75 install in a Duster

451Cuda said
'65-'67 is a little narrower than '68-'70.

Correct. If you can get a 65-67 they would be the best fitment for a Duster/Demon. To be honest, I haven't done this swap to a Swinger/Scamp. I'm sure the offset to the wheel would be more crucial.