Yes, measure the throat on the Supermag and either give Kevin a call when you order or shoot him an e-mail with the measurement and see what he can do to match it.
One thisn Kevin mentioned to me when I ordered my new Arroyo is concerning the optional riser overlays. He said they do make the riser feel a little square in the hand so if you do not want that feel to the riser I would suggest not adding the overlays, they are mainly for looks anyway.
You'll love the 54" Arroyo though.