Bill
Mine is simply powder coated (which has worked great BTW) with black painted framing, so this isn't from personal experience.
However, I've seen many leather covered bulkheads on originals and many well done reproductions, and the frame rails are not covered along with the bulkhead. I understand your desire to cover the shiney SS framing though.
I'd cover the bulkhead separately (I presume you don't have the "suitcase" cutout given your late series model) and keep it as smooth as possible to prevent sagging from our high heat. I'd make sleeves which wrap around the frame and close with sewn in velcro (outer edge/won't show) as some do with the down tubes from the dash to the tunnel.
Presume replacement down the road, so KISS.
