I think that the good guys that rebuild them is steele's(the outfit that does window and door seals)
the other way to address this is cab mount kits are all for a C10 which comprises all 4 cab mounts (best buy) get some C10 frame brkts and do it that way
most all of these restoration parts places have the insulators or mount kits with all new bolts etc.
ron
http://www.steelerubber.com/