I have two C4's for the Cougar... the one that's in it now is the "older" style with a 24 spline small input shaft, dipstick in case, bell housing on front pump bolt circle. I have "spare" on the bench, it's the newer style dipstick in pan, separate bell housing bolt circle... It has the 26 spline bigger diameter input shaft. I wanted to swap to the one I've got on my bench, but I havn't because of my TCI high stall converter... the input shafts are different. TCI can change this, and I'll probably have it done this winter... along with adjusting it to stall higher.
Ron.
PS. I am completely unfamiliar with the C6 or FMX units.