Sounds like you'll be dropping the transmission pan regardless. That is a great time to play with the cable adjustment, as you can see it move and insure that the detents on the internal shift lever match your push-buttons. Those cable-shift transmissions were also shifted with a floor-shifter (my 65 Dart) and a column shifter (my 65 Newport). I have a 727 small-block cable-shift tranny in storage and have seen others for sale, usually at the $100 I paid. I have rebuild kits for all w/ clutch and steel plates which were only $50 on rockauto (come & go). If you do go to a later transmission, you'll need a slip-yoke drive-shaft and insure the engine's crank hole matches the tranny's torque converter. They changed ~1967.
Personally, if I did so, I would go all the way to a 5.9L Magnum V-8 and transmission for it. That would give you MPFI, roller-rockers, crank-trigger ignition (though still a distributor, I know strange), and a 4-spd auto-trans. But, you do need to cut and reform the floor tunnel. A new Hemi is another option or even a Chevy LS. But, first yiew the youtube of 1967 Plymouth w/ slant-six running 9.09 sec in the 1/4-mile. He did add a turbocharger and such.
