Here is the bracket, with tube for the shaft through the windshield:
Looks like the windshield holes are 4 11/16, motor mount 3 5/8.
Excellent! Using that to mark before drilling will help and then you only have to ensure it's level.
But, let me point out - the outside holes, 4 11/16", are bracket-to-motor mounts (reference my pic and your original) and the inside holes, 3 5/8", are bracket to w/s tube.
I'm wondering now, also, where CJ3A's parked their wipers....were they both to the same side? Drivers or Passenger side?
To piggyback on Bob's post, I think the park for the wiper arm should be to the center. The driver's wiper to the right and the optional passenger's wiper to the left. Hopefully, when you power up those donor wiper motors they have opposite parking spots to allow you to copy the originals.
FWIW, I posted on the 3B BB about a seller on ebay was offering NOS wiper arms. I don't know if he still has any.