Overdrive transmission

If you are doing to 518 / 46rh route
I'd also suggest looking into something like this kit ...
PATC 727 to 518 / 46RH Conversion Kit. Click here for 727 to 46RH swap wiring diagram PA 727518 - PATC - TransmissionCenter.net
I have no affiliation with them I was just planning on using this myself.

You will probably need to do (fab or buy) something different with the rear trans mount...
US Car Tool has a modified trans mount looks nice but isn't cheap.

Again no affiliations...