Which starter with 6-speed in NA?
 

Since I will be installing a 6-speed in my 1996, and the starter in the car is the original, I figure while I'm at it I'll change the starter. Is there a mounting difference between the 6-speed starter and what's in my car? I know that one bolt is removed from the starter mount during the swap if I use the NA starter. Rock Auto lists several options for starters, seems the auto tranny starters have more torque.

I wouldn't count on an auto starter to fit on a M5 or M6. Starter motor is such a low failure item, I don't think I'd sweat it.

+1 low failure item in a miata... don't bother. Super easy to replace anyway.