You could locate a driveshaft from an 8&1/4/904 equipped A body (I'm assuming '67 up A bodies have the same wheelbase) or as said (best way) have yours shortened/balanced/good ujoints. With the rear axle on stands (loaded) butt the driveshaft all the way forward into the trans/measure how much too long it is/subtract 3/4"/give that dimention to the shop.