rear disc conversion

Rear drum lock-up ? Not in my 50 + years of driving Dodges. And I have owned over a dozen 69 to 73 models.
Only drum problems ever was on the only one that had 4 wheel drums. Getting off the freeway in the rain. Zero braking until the water burned off the drums/shoes.
Only front disc from now on.
The factories use single piston calipers for their non performance cars for one reason. Expense.
It it cheaper to install 4 wheel disc when the car is assembled. No adjustments needed when installing. All parts are exposed for easy inspection. Simpler mechanism. Less parts.
Braking is not equal on single piston. That is the reason that most older single piston equipped cars have a thicker inner pad when new. Multi piston calipers also used a larger/longer pad since the area of the caliper pistons is increased, your swept pad rotor area is much larger putting more friction to the rotors and allowing for quicker heat dissipation..
https://www.carthrottle.com/post/is-it-better-to-have-more-pistons-in-your-brake-calipers/https://www.tomorrowstechnician.com/multi-piston-brake-caliper-service/