Your front and rear springs need to be matched to achieve balanced handling. I would say that higher
rate front springs would increase understeer. Typical suspension tuning is done with swaybars, once
you have achieved the proper spring rate. Street handling is a trade off between comfort and trying
to achieve grip. A adjustable rear sway bar is probably the best way to try and balance a early mustang.
The "Stam Bar" brand sway bar allows you to have some adjustment, more rear bar give you more oversteer. I would go lighter on the springs and heavier on the swaybars...
1966 Mustang Coupe