Those types of shocks with the coil spring are really designed for carry temporary extra weight, such as camping gear, not extra weight on a continual bases.
The best thing to do is just add another main leaf(leaf almost as long as the main), it will level out the rear, and compensate for the weight of the removable top. It should cost around $35 for a set of junkyard springs, new center pins, maybe a little more if you have to replace the u-bolts. Any set of springs will work, as long as the main is a long enough(after cutting off the eyes), domestic truck springs are a little thicker and wider(measure width before buying), but they will work just the same.
You could also have a spring shop add another main, but that will cost you more money. Might also google "Zuk coil springs", it is a coil spring mod that goes between the bump stops and top of the spring pack, similar to the helper airbag spring.