Hi,
I'm just curious where you are with your 37 Plymouth. I also have a 37 Plymouth P4, 4 dr sedan. I have had it since 1987 and am still working on it. I did keep the straight axle. I put a Buick V6 231 and TH350 trans with an 8 3/4 rear end. Just a cruiser, plenty of power for me. I added disc brakes on the front using Mercury Villager rotors and the original hubs with drums removed. My brother and I designed a mount for Wilwood 4 piston calipers. Works great. I used the front and rear seats (6 way power, leather) from a 2005 CTS.
If you haven't done anything on steering yet, I would suggest electric power steering. A 2010 Toyota Corolla column with the controller worked very well and easy to fab up. I used the entire column and added a set of relays in a box for all turn signal and other switches. It wouldn't be that difficult to graft onto another column. The 2010 Toyota steering has a default mode with no ECM required, just the steering controller. There is a lot of info on the web about it. I also rebuilt the original steering box and will be getting new king pins soon.
I've had a lot of fun with it over the years. Still not finished, may never be, but it's fun to drive.