ok to convert 10 x 1 3/4 drums to 11 x 2 1/2 on '72 dart?

Upgrading rear brakes can also tilt the brake bias in the car to the rear brakes. Which could make them lockup earlier than the fronts, specially on wet roads. You don't want that.

An adj. pressure valve in the rear line can solve this, but upgrading the front brakes would be a better option overall.