help with front coil spring compatibility

[tranz am]

Ok Im on the last stretch of my hunt and need to know if the coil springs for 67-72 are the same as those in 73-87. I cant seam to find a 3 inch drop spring for 73-87.

What am I doing?
Im working on a 69 gmc c15. My objective is to drop the truck 5-6 inches, convert over to power front disk and retaining the stock 6 lugs appearance for the lowest price.
I have converted the 69 front end to a 78. This utilizes the better bushings offered on the later pickups and gives me the all I need to convert to disk 5 lug. This is all great if I wasn't in love with the centerline 6 lug rims I already have.
So I did some searching and I have found a 6 lug rotor that will bolt to a 2 inch drop spindle that will accommodate my stock 78 backing plate and calipers.