I've seen composite leaf spring sets for MGBs at a couple of locations on the 'net. I know it doesn't really help you and your application, but I *think* the pricing was roughly double standard leaf spring packs.
As another option to throw into the mix, I've seen an outfit in the UK (can't recall their name ATM) that makes elliptical steel springs (again for MGBs) that offer a weight savings over standard spring packs, slightly more expensive then standard springs but less expensive than composites.
Sorry, I don't have any info for your specific application, but I thought it might be a little helpful to toss in at least one alternative option...