engine block question



I guess some of this was answered in the previous topic but I'm real new to cars and performance. Ihave a 144 I6 and I was thinking of upgrading. Does engine block determine engine size or do the heads?
I would also like to swap my 3 on the tree for an automatic. Are there only certain transmissions that will mate with my engine? Does changing the heads affect what transmissions will mate to it?

Since I'm pretty sure my engine is not original, how can I find out when my engine was produced? Did much change from year to year when they built these engines?

If it helps answer any of this my car is a '63 falcon.

thanks for any help