Plymouth Valiant model car or model kit

73/74 Duster was made but is hard to find.

That dirt track car or the later LA Dart kit could be the back window/C pillar source.

Rear bumper will be "fun" but is doable, however-

The rear wheel openings will be the hard part.
I tried that on a 67 Charger to Coronet conversion.
It took months and months to do one side, and that's where that project stopped.
Well, that and the introduction of the 67 R/T kit.

I've seen "IRL" people use front wheel openings to patch rears on those cars.