The Mustang conversion would be cool, then since the front of the car is modern might as well bring the rear up-to-date with a modern disc-brake axle. Since the modern axle can handle a lot more power than the old Mopar piece, and there is now a Mustang front end on the car, it would be a shame not to drop a small-block Chevy in the hole with a modern automatic transmission. Of course the steering column needs to be replaced with a tilt unit so it looks better with the Vintage AC system and Toyota bucket seats.
Now you can really utilize your 'old' Mopar! ?
Or......you could just install a disc kit and enjoy your vintage driver. ?
The above scenario is what we call "mission creep". It occurs when we don't establish what we really want from the car and stick with that framework. We see it all the time in the custom-built aircraft community, the basic set of avionics ends up being $70K..........and rarely gets used to its full potential. But I respect everyone's right to decide what they want to do.