Since you've suggested it's 11-inch, Spitfire suggests a starter with the bolts in a straight line will work, I agree, the mounting bolts will be of the same length.
You may want to take off the inspection cover and remove the solenoid from the starter so as one can manually operate, without cables hooked up, and check gear backlash by using a small paper clip. The correct distance between the starter pinion and ring gear with hand-operated solenoid engaged is the paper clip diameter between the two gears, i.e. pinion tooth and ring gear valley. Adjust starter nose with shims accordingly.