A couple of things come to mind:
1)Are you positive both leaf springs are of the same weight rating? Suppliers screw up all the time and could have mistakenly given you 1 for a big block and one for a 6 cyl.
2)With the car on the ground, measure from the floor to the spring mount locations on both sides. If they are different dimensions, it probably means some fram work has been done in the past.
Post back your findings.