Couple of sandbags in the trunk plus some blocks under the rear wheels should do it ... but, even if you get the rebound straps attached, they shouldn't be under tension in normal driving conditions.
I know Bs better than As, but looking at the drawings of the rear suspension on the Moss site it looks like an almost identical setup.
The only advice I can give without getting under it is to check the strap length and spring curvature against the original specs (the Bentley book should have them) and send whichever bit is wrong back with a rude letter. It's possible someone sent you the wrong bits by mistake.
If both are right, then I'm completely out of ideas ...