Need info on Duster

If the vinyl top and trim is there, it's likely original. People usually remove them, not add them. So you should keep it.

As for "frame off resto", it's not applicable to a Mopar. (Someone would have mentioned it eventually.) You can separate the frame from the body on GM cars, but not Mopars, because they are uni-bodies.

Here is a little Duster history - http://www.allpar.com/cars/plymouth/duster.html

Good find and good luck with the resto!