The difference in the flywheels could be weight? I have heard that flywheels will vary from about 26# all the way down to about 20#. Weigh a couple of yours and see if there is a difference. I have several and will weigh them when I can get them out. The F-85 automatic flywheels are different part numbers in my 1963 parts book for a 2bbl and 4bbl set up and I think it is weight.