rear leaf spring problem

Hi all. New to the forum today. I have nearly finished the complete restoration of my '74 MGB (chrome bumper). I replaced the rear springs with new ones from Victoria British, and the car is way too stiff. The springs have 6 leaves which I believe is correct, but I had to put about 600 pounds of weght into the trunk to be able to attatch the shocks and rebound straps. The shocks are fully extended, and the straps are taught. The car has no bounce. I have not tightend the nuts and bolts yet, so the suspension should be able to settle to its natural state. Any ideas what could be wrong? thanks.