There would not be a difference between Dodge or Plymouth (or Chrysler, as far as that goes) BUT it does make a difference between years and A/C vs. no-A/C.
All F/M-body A/C cars have a break between '76-79 and FMJ '80 to '89. The Firewalls are different as well as the angle of the A/C expansion valve (the '76-79 A/C cars, the expansion valve sits sorta straight across where as the newer expansion valves sit at a steep angle.
A '76-79 A/C firewall
Also notice the duct that connects the blower housing to the heater/evap case.
'80-89 A/C firewall
There is no duct extending into engine compartment (photo kinda cut that off, though). Also the heater hoses are side-by-side instead of top/bottom.
The the non-A/C firewalls were the same from '76 to '80 and maybe later. Not enough '81 and newer no-A/C cars in my parts to look at.
I can see three different firewall pads:
'76-79 FM A/C
'80-89 FMJ A/C
'76-80 FMJ no-A/C (and maybe later)
I'm not sure about the newer no-A/C cars - so maybe a forth design.
BudW