If you can measure the diameter of your rear wheel cylinders, and reduce the size 1/16"
it will probably eliminate the lock up situation, assuming the brakes are all adjusted correctly. If 1/16" is not enough go to the next smaller size. Wheel cylinders are cheap and is the best way to balance the brake system. Proportioning valves take away brake force, not the most desirable situation in a panic stop. Here are some popular sizes and Wagner part numbers for Ford 8" & 9" rear ends, for both the right and left sides.
7/8"- WC 17507/8, 29/32"- WC 28804/5, 15/16"- WC 17509/10, or WC 59240/1, 31/32" WC 49234/5, 1"- WC 78974/5. If your parts store doesn't have Wagner have them cross these numbers over to the brand that they carry
HemiDeuce..