Jordan, great question!  I don't have the answer, all I know is that my two Indian gearboxes, a '46 Chief and a '29 Scout, are close (or even the same) inside...without doing any measuring.  And outside, again without doing any measuring, appear to possibly be interchangeable (barring the difference in drives..chain versus gear).

I'm going to get out my copy of Jerry Hatfield's guide and see what he says on the matter!

Hi Jordan, sorry not to reply earlier, having difficulty accessing website.

The scout and chief boxes are not interchangeable, full stop.

The earlier scout trans and the 101 trans will both fit on the motor, the mounting ears are the same size, but the primary cases are different due to the larger clutch, and so the 45 and 101 trans cases from mid 1927 have a larger circle where the primary case mounts.

If you can bear with me, I will try and post some pictures and dimensions later today which will make it a lot clearer.