An update warning- If you plan on using the factory flexplate be advised that the old school starter bendix strikes the tone wheel preventing the car from starting!
I think Chrysler moved the starter location on the A-500 and 42RH (overdrive 904 units) so the same flexplate could be used with the 518/46RE.
The mini-starter I have here will not fit with the 904, there is a section of the starter the strikes the block. There is a large "hole" at the rear near the oil pan rail, looks like 1/4 of the edge has to be removed.
The mini-starter does NOT have the large bendix like the old school starter does. I think I can grind off a section of the block and it just might work.
Otherwise you have to pull the motor out and have the tone wheel machined off.
What a pain..