Those draft feet are big, aren't they? I shod a Belgian mare in school that wore size 7 shoes - you had to put one side in the forge at a time, because they wouldn't fit!
I also agree with finding some live sole, even if it is just at the toe quarters, just to use as a guideline. Flat feet often times equal flat coffin bones (often associated with thin soles), so I would not pare away at the sole to make it concave - just find what's live and move on. I might also leave a freckle of length on the quarters, then rasp them thin when you bevel from the top. This way, he can wear away the quarters on his own, as opposed to being set down ground right away. I have found that this helps my paint with extremely flat feet/coffin bones.