A keelson will really help this boat . It is a work horse and from what you describe, the hull needs some correction. I have an EM White Sportsman
ca. 1905 . It is a heavy water, hunting boat and was fitted with a keelson originally running from stem end to stem end. It is oak 1 -1/8" x 3/8" and riveted with hand made copper rivets at every other rib , peened over a copper washer with the heads countersunk in the keel and bunged with an oak plug. The hull midship is " some wicked rigid ", and ready for moose quarters. You will not soon forget the sound of all that peening .
Have fun
Dave