I just finished a C4 trans for a buddy with a 82 mustang. He is swapping in a really stout 351, so the “original” trans wasn’t going to make it.
I say “original” because it turns out the trans is a 69. Everything was going smooth until I tried to put the selling rings on the pump. They wouldn’t fit, they were too big in diameter, and too thick to fit in the ring groove. After doing some research, I found that Ford changed input shaft spline count (1st design, 22 spline on both ends, then a 22-26, and finally 26 splines on both ends) which resulted in design changes to both the pump abd the forward clutch housing. That meant his converter wouldn’t work. He bought a 71 core (checked the date codes this time) and everything went together ok.
I found out there is a case with and without provisions for a dipstick tube (some transmission have the full tube in the pan) cases with or without vents (some transmission use the case to vent, others have a vent in the extension housing) different valve body designs, different return springs in the clutch drums, different vacuum modulators, countless servo calibrations, the list goes on. And that’s just the C4!! There are lots of the same kind of production changes to the C6, the E4OD, AOD, etc. All these transmissions had a fairly long production life, it seems to me they could have easily perfected ONE design early in its life, and then kept it.
GM has a lot of stupid interchange problems as well, why different bellhousing bolt patterns? Internally/externally balance, different flywheel/water pump/ balancer combination, but nothing like Ford. I suppose if your around it for a long time you get use to it.