Tony, I don't know what the original reason for starting your disassembly was, but you can escalate this into a full "might as well" project really fast. Just a friendly caution. I wouldn't do the oil pump FWIW. But you might as well do the rear main seal. D'oh! See what I mean?
I can see em now. The 56 is a better engine, and hey...it runs! There is no such thing as a numbers matching 55. Yes, I realize they could see that it is a 56, but these numbers are hard to read anyway with everything bolted up.
Usually a leak from that area is from them not realizing there is already a gasket there and installing another one. Two don't seal better than one. Axe me how I know.
If you use NOS filters and the gasket has been in the box for 40 years (curved around the filter), it probably won't straighten. For now, you can make a gasket out of the blue gasket material (1/8"?). Then probably try contacting NAPA and seeing if they can get new ones.
First off, I agree with Pete's diagnosis and fix. The only other thing on a door with good hinge pins, would be if the weatherstripping is new AND / OR installed backwards.
Jumping out of park sounds like trans mounts (Like Joe said, there is a thrust pad and trans mount) AND / OR linkage. Both of which are adjusted with the trans in the car. That is where I would start.