You can google some 46rh swaps and it details what was cut and how to change the rear mount of the trans. I think someone was making a cross member at one time. Might be a little tighter in a 64 A body but well worth it when its done.
Can you tell me a little about the Rear Mount you are referring to? Are you referring to fabricating a new mount or changing out a part? I do realize I will have to shorten the drive shaft.