1966 plymouth valiant questions

Will the front and rear seats from a 4 dr interchange with a 2 dr.
I'll assume that you're asking about front bench seats. They will fit, but they are different. The 4 dr. front bench is not hinged and does not fold forward, eliminating back seat access when installed in a 2 dr.
The 4 dr. rear seat bottom is more rounded off on the front corners, and the 2 dr. seat bottom is more squared off to meet the sides. Using a rounded off four door seat in a two door will leave gaps at the front of the seat on the sides.
Will front and rear glass interchange.
ALL early A body windshields ('64-'66; 2dr/4dr/convertible/wagon and Dart/Valiant/Barracuda) interchange.
All rear glass is different, must match same to same- i.e. 4 door to 4 door, 2 door hardtop to 2 door hardtop, 2 door sedan to 2 door sedan, etc. Dart rears do not match Valiant rears.
Do they make front and rear glass rubbers.
I have not seen any new early A rear gaskets being made, other than the Barracuda. Try to save and reuse your old one if it's in decent shape, otherwise you'll have to make a new one out of bulk gasket material- save a piece of your old one to try to match the profile.