I don't know about your transmission but the 3 speed on my '35 was very easy to rebuild. I just disassembled it (watch out for the many needle bearings) and looked to see the condition of all the parts.

I ordered a small parts kit, and gasket kit from Bernbaum and bought a new second gear on E bay. That was all mine needed.

There is not much to the one I worked on.

Don't know much about the Plymouth trucks but a quick look at the 1936-48 Plymouth Parts Book shows the same number for the four speed "transmission assembly" for all years of Plymouth trucks. So if you are looking for parts, a four speed from any other Plymouth truck should be a source. Looks like a number of the internal gears have pretty low part numbers (5nnnnn range) which means they were first used prior to 1933. So I would guess that some other Chrysler built vehicle (maybe Dodge trucks) used common parts to this for a lot of years. That bodes well for finding stuff to bring yours up to spec.

Slowly but surely taking apart, cleaning/repairing this pick-up. Yesterday removed clutch/brake pedal assy. and the 4 speed and driveshaft. I have never had the truck in running condition so I don't know what to expect from the drivetrain, especially the tranny as this truck has several signs that it was worked hard I want to know if anyone has had one of these transmissions gone through and what parts or service it needed. Information I have is the ratios from the commercial vehicle flyer but not much else, I have not thoroughly cleaned the case of it but so far all I see for identification is the NP gear circle logo cast into the side near where the power takeoff plate is. Thank you.