The 50 Ply. parts book only shows two different part #'s used. Stock 1" dia. shaft and a 1 3/8" shaft heavy duty with NO model breakdown per application. So, either should fit all models.
Just suggest using a heavy duty cross over shock for 50 Plymouth.
Have found Monroe 3100 or 5752 May be what you need.
Monroe has a website and look around their site and find the spec. page . gives all open/closed measurements, mount type, size of the mount, etc.of all the shocks they make.
Then do a search for that make and # for the best price avail.!!
Best.
DJ