Probably a lot easier to buy a complete modern retro guage cluster from some place like
United Speedometer [img]
http://www.speedometershop.com/images/encore.gif[/img]
These guages come all properly tweaked for a small block and 12 V negative ground for your '53 GMC (originally six volt positive ground). Since you likely have addressed the speedo, only the other cluster is needed and that's reasonably affordable and you can turn in your core for credit.
This is a bolt in option and solves all the problems in one shot.
