I'm not trying to be a smart guy.
Since the original was mounted below the floorboard I think the correct answer would be, "it depends on what brand/type/manufacturer master cylinder you wish to mount on the firewall". To me, modifying brakes and steering on these old trucks need to be done professionally or at minimum buy a kit from an established and well known parts provider. I don't want to be the one to give advice that could lead to someone getting harmed or sued.
