The 30s Mopar flat back sedans did not have a trunk lid in the back. Originally, the spare tire mounted on the back panel (I had a 35 Plymouth 2 door sedan). There is no structure there other then the minimal support for the low mounted spare tire and a bit of rear window reinforcing. If you want to add a door on the back, you will need to build a complete door jam, and a complete door frame.
A 35 Plymouth car is pretty narrow (37 they started getting a bit wider, but only by a few inches). The widest point of the body is at the rear of the front doors, and the firewall is several inches more narrow that at the rear of the door. A pair of bucket seats will sit against each other in the center, and you need to watch the seat width if you want to close the doors!

Lots of great ideas. Some cool looking rides, but, all of the extended cab trucks, lengthened doors, modified body shells and what not, all require structural body work, and that either requires existing skills, or a willingness to learn those skill, or knowing someone that has those skills.
The old cars & trucks are fun, and they do present a clean page to make them something as different as you wish, but fitting modern drive trains, adding modern driver conveniences, and adding extra room inside all require a lot of work that someone has to do, or have hired to do. How bad do you want it? Gene