There has to be a difference in spring rate. It's a ~200 lb difference. My front sits a tad higher with BMR springs than 5.0s do with the same springs. They're very close to factory 5.0 spring rates, so shouldn't be very different of a result.

 

I would expect that if you did a straight swap of the fronts you would end up sitting higher, due to the difference is nose weight of the cars, no?

 

S550 Mustang Springs

docs.google.com

I think I found it. Try this

 

I believe different spring rates, but I can't remember how much. There is a chart somewhere over on the 6g forum that might help you. I remember looking at it when I got lowering springs. They had the spring rates of all the different springs (oem and aftermarket).