Hello Ian...........welcome.
You would have to either make a rear seat, or find something from another vehicle that would fit.
Not impossible, but you have to go to a little effort to accomplish.
Tune-up parts, and other things are available - some thru your local auto parts store, some thru e-Bay, and
some thru certain old MoPar vendors.
There are some members here from the U K with Plymouth P15s who will hopefully chime in and give you some further information.
carnut.com has some pictures of street rod type Plymouths. Link to the index.....http://carnut.com/photo/list/list.html