I know with the W123 peddle assembly, the difference between the auto and the manual assembly is the shaft the peddles hang on. the shaft is longer for the Clutch peddle.
The pictures below will show what I mean.
The aluminum mount is the same on the two.
maybe the peddles can be swapped over to the 126 mount bracket? I know the mount bracket is different on the 126. then you wouldn`t have to modify the mount bracket.
This is just a thought.
I think Sirnik84 modified a 123 assembly to work in his 84 300SD 4-spd swap.
Charlie