flywheel ring gear offset is a factor making the starter transmission specific. here's a post by RobbMc himself regarding the various offsets & fitments
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Ford starters are a little complicated. I started working for Tilton in the 1980's and have been designing starters ever since and I still get a little confused at times.
Since the 1960's, most Ford V8 engines (except maybe the new modular motors) use one of three basic starter mounting designs .
The all use the same 9 tooth, 1" diameter gear.
Some use three mounting bolts (such as FE) and some use only two, but fortunately 2 of 3 mounting holes are in the same place so they can be interchanged.
There are three different ring gear offsets (the distance the ring gear is from the starter mounting surface).
SB's and Clevelands with automatics use an offset of 3/4". All FE engines (as well as 5.0L with T5 trans) also use a 3/4" offset. The starter register diameter (ie. the diameter of the hole in the starter mounting plate) is 4.09".
SB's and Clevelands with a 3 or 4 speed manual trans use a 3/8" offset. The starter register diameter is 4.14".
351M/400/429/460 engines use a 5/8" offset. The register diameter is 4.09". The starters for this application are easy to spot as one mounting hole is tapped.
Yes, there are several different ring gears with different tooth counts but this is taken care of by the starter mounting plates. The starter mounting plates for the larger ring gears have the register diameter further from the center of the engine.
I am sure there are exceptions to all the above, but these are the general rules for Ford starters.
If you measure the distance from the mounting surface to the ring gear and measure the starter hole in the mounting plate, you can usually determine which starter you need.