Had mine built at the Roadster Shop w/6" Z in the rear. But they're fabricated rails, which I don't really like. I'm sure any number of people can build a frame from American Stamping rails and Z the rear. It's not a big deal.
Kiwi Konnection should be able to, depending on what you want. (661) 871-5494. He will be at the Hot Rod Reunion. Or I can but he is does that every day. He builds ch***is's every day. Rodger
I have got a big truck frame begging to be built with a killer Z in the rear. May have to go ahead and do it. It will be for sale