Overdrive in Early A

Here’s my two cents:
If you are planning on an A833OD—which I myself would recommend over any other trans for ease and for awesomeness, it’s straight-forward.
The ‘64-‘66 (and later) A-bodies came with the old iron A833 which are the same size, so the only cutting necessary is for the shifter. Brewer’s has a fiberglass tunnel piece for that, but a real steel one would be better IMHO.
If you have a pre-‘66 car, you would need to get a new driveshaft, etc., as the older ones were ball-and-trunion. You would also need all the parts like the bell, clutch, as the ones from a 903 won’t work. Obviously, if your car is a 904 car, you’ll need the pedals, Z-bar, etc.
‘66 cars began the use of conventional U-joints, so an easier swap.
The trans you need is from a late A-body or an F-body.
I had a ‘63 Signet that had this done (PO) along with a 2-bbl. and it had 3.23 gears. The car was awesome and it got 25 MPG. I plan on the same combo for my ‘66 Dart 170 postie.
I hope this helped, and if I gave any info that wasn’t spot-on, I welcome folks who know more to correct me for the sake of accuracy.