I think you are describing those long rear shackles that were in vogue in the 60's and 70's. If so, there is a reason you don't see them much anymore.........they don't work well. I'm not sure how to get a monoleaf to set higher, but I'm pretty sure you have seen why the shackle route won't be the best solution.

Some other, more knowledgeable Chevy guy on this forum, will have a good answer, I am sure. But I would bet money the shackles have to go. I remember seeing cars with these where they had bent sideways from taking turns.