Is there any mechanical difference (specifically mounting characteristics) between the 752 traction motor on a Dash 8 and Dash 9?
Can't say, but I would think the basic mounting parameters between the Dash-8 and Dash-9 would stay the same.
What I do know is that you have to be careful about GE part numbers. The traction motors would actually be a 5GE752(suffix). 5 is GE's parts series for heavy rotating electrical, just like the FDL engine was actually a 7FDL(cylinders)(suffix). The suffix letters/numbers are tricky, in some cases they can be a simple upgrade or customer requested modification, or they can be a different gear ratio, to a design that is pretty much incompatible with anything else. Unfortunately GE did not do a one size fits most parts catalog like EMD, and you can't always tell which part numbers are interchangable.