4 speed Manual transmission linkage
 

on the three piece iron bodied transmissions. Is the linkage the same for each different design. Flatback vs. Slopeback design?

I don't know. If you know which cars came with each transmission, you could look up the part numbers to see if they are the same.

As I remember, there are two different configurations of shifting arms on the iron box. One has a splined attachment to the shift arms the other has two flat sides (the 3 shafts that the arms attach to) and the arms slip over and are bolted on. That configuration MAY use the same shift rods as the one piece box but I dont think the rods from the splined ones wont work. You will have to investigate further to be sure. I no longer have any trannies lying around.
My guess would be they are not interchangeable but you never know, like I said, they might be. Hopefully someone will know for sure.