A friend put the Chevy 8 speed in his 68 Dart, with electronics was over 7 grand. I would look into the 727 and the over drive that bolts to the tail shaft (can’t think of the company). Also have to redo the rear trans mount to even do the factory od trans. Copes trans with adapter might be the best, they all will need an adapter. I’m looking to do same in my 79 truck, I have a 92 trans that I’m going to cut off the front part and weld the big block piece to it. like the factory SHOULD have done. I’m measuring now to see where the two cases match up. Even thought of using the Ultra Bell bolt on housing.(cut trans housing off and then it bolts to the front pump)
Look up PATC they have the wiring kit you need.