Brakes Junction Block

99 Cobra is hydroboost and ABS. The fitting style and block is not the same as the fox body Mustang. It's more work to do this vs a simple 3-2 conversion for the 1993 cobra MC you are purchasing.

You need an adjustable prop valve regardless. The valve controls knee point to the rear brakes which is the point that pressure increase to the rear calipers occurs at a slower rate than the front calipers.

Things can vary bias are weight of car, weight distribution, tire sizes, etc, so you'll want a little adjustability. Ultimately what you are trying to avoid is a worst case scenario of panic braking on the highway on loose surface and locking up the rears.