Better idea: Raise your transom. Some shallow sided Johns have done that in the design of the boat. Look around the net for such and get some ideas. Not that big of a deal. Get 2 pieces of aluminum plate, 0.100" or so, one for either side, some plywood for the center section, Handfull of bolts and locking nuts and some marine sealant and make your 5" extension. I'd go down the existing transom at least 6-8 inches. If you have a knee in the way just cut a grove in the interior sheet around it.
