Can anyone help me with what type of starter do I need to fit 1957 D2 4U series with D311 motor so I can bypass the pony motor.
It will need a Helical starter pinion drive I believe its a 11 teeth 39MT 24vt I would need can anyone confirm that I have this right
and does anyone know who sells these starter motors Cheers Matt
 

The preferred starter motor is a Delco 42MT but there are complications when setting up a later D2 that has the provision for direct start.
The original inertia type starters had a smaller diameter helical gear than what is available now for retrofit. To make the current available parts work the starter (big hole) needs to be offset ground about 1/16" or the drive end housing nose offset machined. I'm not aware of anyone supplying units ready to go unless you are starting from a blank flywheel housing boss.