The 7646 movement is an old school movement with jewels and it is pretty hefty. in height and diameter. The only successful transplanting of the 7546 was into a 7002 diver case.
I currently have a 7C43 on the bench completely torn down. I also have a beat up 7C43 case. I have plenty of good 7546 movements in my movement storage bin.
When I ger a chance I might just look and see what if anything fits. Some type of movement holder would probably have to be custom made but that is not to hard.
I have an old set or word out 7C43 hands. I can pull a spare 7546 movement out and see if they fit.