Proportioning Valve Differences

[70 Challenger Lover]

Helping a buddy convert manual drum brakes on his 70 Challenger to one of the inexpensive disc brake kits with booster.

I told him that it's recommended to also switch the old proportioning valve to the disc brake valve during the conversion but I did not know the specifics of how they differ. Only that the factory had different ones for different applications.

He intends to try it with his old drum brake valve and he intends to add in the adjustable valve provided in the kit to reduce rear braking pressure if needed.

As far as I know, he should be okay. Are there any other differences, besides front to rear pressure differential, that we need be concerned with in reusing the drum brake valve?