new rear disks, need more "bite" - any suggestions?

The problem could be the wrong proportioning valve, or the Ford calipers need a larger volume of fluid than your master cylinder provides. There is a balancing act between the right amount of fluid volume and the right amount of line pressure. Sometimes when you swap in something quite different than what the original system was designed for, you can run into these sort of issues. Baer and Wilwood both make a brake bias adjuster valve that allows you to dial in the front to rear balance. You might consider adding something like that.