All other things being equal your weight will go up, not down as you shorten the riser. Unless you increase the limb pocket angle or shorten the limb wedges, shortening the riser will cause the bow to stack more at longer draw lengths. With that long a draw I would stick with the long limbs-long riser combination. In most bows shortening the amo length by 1 inch will add 2-4 pounds of draw, so if the 66"amo bow with the long riser yields a 50# bow at a given limb thickness, reducing the riser by 4" assuming you dont change the limb pocket length will yield a bow in the 58-66# range. If you are dead set on shortening the riser try shortening the limb wedges by 1-1.5" each to accomodate your long draw. In my experience cutting an inch from each wedge will reduce the draw weight by 7-8 pounds on a recurve. Elmont Bingham will be able to help with this as well.