I'm running the original drum prop valve (and master cylinder) with front discs as well. Been that way since I did my disc brake swap about 160,000 miles ago. The car has been my daily driver for the past 8 years and I drive it in all weather, including the occasional snow and ice we see a few days a year in GA. I run 195/75/14 tires all the way around, and I've never encountered any issues with my rear brakes locking up. For a sunny day driver I don't think you'll have any issues just throwing the spindles on and calling it good. The disc brakes are without a doubt the single best thing I've ever done to my car. They have enabled me to avoid several accidents, makes the car far safer.