2-cable 904 trans swap

I have a 63 Plymouth Valiant with the original pushbutton 2-cable shifter on a 318/904LA.I have a few problems with the transmission:

1. Parking lock sprag is stuck in neutral. Even with the cable disconnected, the sprag is locked in. (wouldnt be bad if my e-brake worked)
2. The shifter cable hole in the case is too large for the o-ring to seal, creating one heck of a mess.

I like the idea of the original pushbutton selector, but this transmission hates me and has vowed to never let my car see pavement again. I am ready to swap it out for the easier fix and sell it.:banghead:
Sooooo my question is :
What all is involved in swapping over to a newer 904 automatic with a floor shifter from, say, Jegs or Summit?
I have aftermarket throttle and kickdown cables on it as the original engine was a /6. Also, the trans has the flange-style yoke that bolts to the driveshaft.