To the best of my recollection, any Healey overdrive unit will fit on any__side or center-shift__Healey XMSN that has the ovd-specific mainshaft and adapter plate.
It has been my most recent experience that the 100/6, specifically mine (5/14-15/1958 build date) requires an overdrive with exposed solenoid linkage. When I attempted to fit an overdrive with an enclosed solenoid linkage, it did not clear the chassis.
I'm not sure when the change-over date was, but it would stand to reason that cars before that date *might* run into interference there.
This is an early style with exposed linkage (solenoid not yet mounted).
Another 100/6 unit.
I'm at a momentary loss for a good picture of the later type overdrive (somewhere, I have a picture of the fresh Quantum Mechanics ovd NOT fitting into my car...) but here's a bonus TR3 trans with a very similar overdrive; primary difference is the tail housing/mount, but many of the internals interchange, though they may be of different ratios, so some components would have to be swapped in sets.