I don't know how much of a market for a leaf spring frame would be, but here's a Kirkham being built with one.
http://kmsac.com/index.php?option=co...id=55&Itemid=2
I think that the leaf spring frame is an expensive option with an unspecified price for both Kirkham and Shelby. Being able to supply a leaf spring frame at a reasonable price might put you in an interesting position in the market.