Your frame should have a bracket with a shaft that sticks out for the shift linkage for an automatic. The bracket should have been torched off during assembly when the factory was building the car as a 4 spd. I have a 64 ss and the frame has a small hole where the bracket was torched off. It is a Janesville built car. As I recall, all frames came with this bracket then they removed it for manual trans cars.
Also look for the clutch linkage cross-shaft outside frame bracket. It should have three attaching weld marks through the middle of the bracket. Also your floor should be modified for the floor shift cover attached on the left side of the trans tunnel. If it only has a small hole cut just for a shifter, that might be a clue. Starting in 1964, they just cut a small hole with a torch, no more pans screwed to the floor. Other members may have better info and pictures. Sounds like a fun project. My last frame-off was a 64 2dr Impala, bench-seat 4spd car. Just finished it then it was stole and never found, 10 years ago.
Have fun with your build. Mine was a lot of work but fun. Your car looks better than what I started with. Good luck, Jon
Here are a couple of pics that may help. Our car is a 348 manual trans, 3 on the tree. My dad converted it to a floor shift sometime in the mid-60's but left the column mounted shift levers on the car (wired tied out of the way...)