General rule of thumb, as you increase spring rate, you have to increase dampening.
This is a typical problem for the HP challenged imports that cut their springs and keep the stock struts and shocks. Cutting the springs increases spring rate. The stock struts and shocks are unable to compensate for the increased compression/decompression forces necessary to control the suspension. Hence the freeway bounce.
Increasing the spring rate on the Mustang beyond what the dampening rating is for the shock will have the same effect.
750s are a pretty heavy race spring. You could drop to a softer spring rate (~600) and be fine. I don't have experience with the Grab-A-Track shocks. I've run KYBs, Tokiko's, and Koni's. I'm sure the Koni's can handle the 750 springs. Not sure about the KYBs but it is a good chance they will since they are a gas pressurized shock.