First things first....
How much clearance do you have for a solenoid mounted starter?
I know it's been done on the 955 but I seem to remember clearance issues.
The for sure fit is the original inertia type starter but they can be hard to come by. These have a remote mounted solenoid and are 24V.
To do the conversion the 42MT starter takes the least room. It needs to be a right hand mount version, clockwise rotation. No matter which starter you get you most likely will throw away the straight tooth drive and drive end housing to be replaced by a 11 tooth RH helix version and Delco #1988748 drive end housing.
Thanks for the reply OM. You always seem to know this stuff. I was a little worried about clearance. There was a post on here recently about putting a inertia style starter off of a D6 on a 955. I wouldn't even know whhere to start looking for one. I have been able to locate a few 40MT starters around here. I don't know if they are any smaller than a 42MT or not. They are all 11 tooth but I am sure are straight.
Thanks again.