Prop valve ?

It will be the same.* The prop valve is there to proportionally reduce the hydraulic pressure on the rear brakes so they don't lock up. For '73 the prop valve is combined with the distribution block and the warning switch.
Booster will assist the push on the master cylinder. In other words, it will be easier for you to push brake pedal.

A few things you might want to do if its better braking rather than easier braking you are after. Some of these will make it easier braking as well.
Make sure the rear brakes are adjusted well.
Fresh fluid and the front calipers are working free
Look into alternative lining compounds that are 'grippier' than what you currently are using.

*1973 Chrysler and Imperial Brake Reconditioning Repair Guide from the Master Technician's Service Conference Session 309