70 duster SS Springs

Hey all i am needing some advise about ordering some new parts for my duster. Im stuck on what to order. One article i read was that SS springs were made initially for A-Body cars. But on mancini racing it says that the SS springs are 20" front and 36" rear section. But i just measured my car it is 20"front and 35" rear. Is the extra length in the rear due to arch in the spring? Will it line up in the original location as my factory springs?

These are the springs im looking at.
Mopar Super Stock Spring Special - Right & Left

Any help would be greatly appreciated.