I can't help with dimensions, but a local guy has a '32 coupe that's a "resto-rod" and I know he's said that the Plymouth Owner's Club is pretty active. You might find something there? Is the seat mounted on sliders or bolted solid? If all else fails you can get a pretty good idea of the seat position relative to the B pillar from that photo of the OEM you posted, scaling off of actual width of the pillar relative to the spacing from the pillar to the back edge of the seat bottom?