If you do not have a Digital Caliper ( http://www.amazon.com/s/ref=nb_sb_no...ital%20caliper ) get one. It makes camera work much easier.
I too would modify the holder, not the camera, provided the holder has sufficient material and will not be weakened by the reduction in thickness. I do not currently have a 2x3 Crown to check how much could safely be removed from the back.
Large Format Film Holder specifications, http://home.earthlink.net/~eahoo/page8/filmhold.html , for 2 x 3 is the front surface of the film is to be .197 inch from the edge of the holder ± .007 inch.
I too have measured the backing plate on my Graphic roll film holders and found a similar variation as wombat2go but all distances are within film holder specs.
Film flatness is only an issue when doing critical work such as architecture or close up/macro work. If you take a roll of film and run it through a Graphic RFH without pin rollers and one with pin rollers the film bows less at the center of the film gate on the holder with the pin rollers than it does in the one without the pin rollers.
See http://www.graflex.org/helpboard/viewtopic.php?t=6391 for tips on Graphic/Graflex RFH problem solutions.