I did a similar swap in my 1964. It had an original thrift master six with a 3 speed. I put a 350 from a 1975 in its place.
I know the 3 speed will bolt right up to it without moving it at all. I also used the same clutch assembly (I was not worried about racing). I can't remember but if I recall my 350 was from a manual truck so it's existing flywheel worked.
I used the motor mounts from a 66 V8 that was in a boneyard.
You will have to do some exhaust work one way or the other. I just ran a new dual exhaust.
I tried running the original radiator but didn't trust the cooling capacity so before summer came on full I updated to a V8 radiator.
Depending on the motor that you are putting in you will have to modify some of the electrical. Mine had an HEI so that was part of the process.
Also, if you have power steering/brakes you will have to account for it as well.
That sounds like a lot and it is. But it went pretty fast and I liked the swap.