David, great to know this, I only meassured some M1 boxes during I repair them.
So I see that the shaft is longer and maybe the theet from the clutch are also diffrent. The Gearboxes from the M1 I rebuilt in the past have also diffrent gear ratios, maybe because of my customers are racer, so they changed some ratios.
Good to know that it is only a small thing to covert them to a Pantera .
Have a nice day, sorry for my terrible english.