Do you have subframe connectors? That will stiffen the car up more, but it actually makes the stiff springs feel better because your car won't rattle and clatter as much over the bumps.
The Ford C springs are probably the ones you have as they are setup to use a panhard bar setup with relocated spring perches I believe which lowers the rear the rest of the way. That is why they are sitting higher in the back. You can cut a half coil off of the rears to even out the stance by the way, and there is even a piece of paper that comes with these springs that states that.
As for Specific or Progressive - The factory springs are progressive rate. The Ford C's are specific. A progressive rate spring will give you a softer ride, but will sacrifice some handling over a specific rate spring and vice versa. Progressive rate springs get "progressively" stiffer as the springs compress. Specific rate springs keep the same stiffness all the way through their compression.
I'd try some sub frames if you don't have any now and see what that does for you (Maximum Motorsport full lengths are what I'm getting - NICE). A strut tower and lower 4 point brace will help as well.
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