You have to remember that you are comparing apples to oranges with you old springs and the Ford C's. The C's are a specific rate spring and the factory springs are progressive rate. The C's are shorter (uncompressed) because they are stiffer and you car won't compress them as far as they did the factory springs.
Say your factory springs are 18 inches uncompressed but are a weak spring. You put a 500lb weight on them and they compress to 12 inches (just making up numbers here for explaining why the C's are shorter uncompressed)
Now on the C springs they are only 16 inches uncompressed but are a lot stiffer. When you put the same 500lb weight on them they compress to 11 inches (more made up numbers). You see what I mean here? The uncompressed length of the spring doesn't have much to do with it when you are comparing springs of two different spring rates. I have the Ford C's and just so you know I'm not pulling your chain mine were the same. About 2 inches shorter.
I haven't done bushings yet so I can't really help you there.
Good luck!
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