When I did my front disc conversion on my 69 C20 I changed out the front springs. Never did anything with the rears. I installed some HD springs (P/N 2772074) bought from NAPA. Tires didn't rub at all. They were 285/70-17 tires with H2 Hummer wheels. The ride was a bit stiff, but hey it's a 3/4 ton truck. Here's a comparison of the springs:
The stock HD C20 springs are as follows:
http://www.napaonline.com/Catalog/Ca...752_0162319756
Part No.: 2771752
No. of Coils: 7.92
Free Height: 12.94"
Spring ID: 3.68"
Spring Load Height: 10"
Design Load: 2950
Spring Rate: 1016
Wire Diameter: 0.812"
The springs I put on from C10, C20, C30 from 1973-83. They are as follows:
http://www.napaonline.com/Catalog/Ca...074_0162301536
Part No.: 2772074
No. of Coils: 8
Free Height: 13.61"
Spring ID: 3.676"
Spring Load Height: 10"
Design Load: 3841
Spring Rate: 1060
Wire Diameter: 0.83"
You could also use C30 mid '80's 1-ton springs, but I thought they would be too stiff. Those are:
http://www.napaonline.com/Catalog/Ca...085_0162304063
Part No.: 2773085
No. of Coils: 7.93
Free Height: 13.34"
Spring ID: 3.65"
Spring Load Height: 11"
Design Load: 3289
Spring Rate: 1318
Wire Diameter: 0.906"