yes, i did that on my old chevy. i didn't really know what i was doing, but i double stacked the 2 or 3 longest springs into my existing pack. it lifted the rear about 3 or 4 inches and hauled the world without sagging. i used a 5/16 bolt for a center pin and made some retainer clips to hold the pack together. i got the extra spring packs and u-bolts at the junk yard for next to nothing, maybe $10, i don't even remember.

the only problem was it rode like a mother****er when unloaded and didn't have enough brakes to stop it when loaded. other than that, the springs held up fine.

having done that ^, i recommend a set of timbren rubber springs for extra load. i've got them on the dodge. last weekend i stacked wood in tight and rounded it all the way up and drove like it wasn't there. very very little sag.