Hey Fred, quick question: if you are running a 15x12 steelies, wouldn't it be easier to keep the 57" wide Ford 8" that you have and just get another set of custom 15x12 inch rims made with the appropriate back-spacing to get the look of bonehead's truck?
Whether you drop from 57" to 53" (for example) won't be a big difference in the look IMO. If you spent $100 a piece through Wheel Vintiques (or another rim company) and got custom back-spaced steelies, wouldn't that work for ya? That 57" rear might be a nice width for you in the long run.
Good luck man!
YB