Inky,
I have a 57 Suburban thats all original. The rear measures 60 inches from wheel mounting surface to surface. I too am looking for an upgraded rear for mine. The s-10 is too narrow from what I can tell its 54 in. I have a 69 Camaro and its rear and its exactly the same dimensions(60 in.) I know this doesn't solve the donor vehicle idea but I think it will be hard to find one thats perfect. I'm looking into an IFS front to go with the rear instead of the donor idea. Hope this helps.