OK...Why do you need 3/8" shafts then? If the existing motors are NEMA 34 then replace them with NEMA 34. If all you can find is a 1/2"shaft, buy a new motor pulley or bore out the old one. If you go to NEMA 23 motors, you'll have to make new motor mounts.
Personally, I give it a shot with the old motors. It won't cost you anything to try. If they don't work, then replace them. You'll use the same drives, power supply and BOB regardless.