Got home in the late afternoon yesterday and jumped on moving the 4'6" forward. First off I measured the reinforcement block and cut it in half (longitudinally).
Next I transferred the outline to the board, and proceeded to cut out the opening for the blocks.
When I get a chance in the next couple of days, I'll enlarge the openings to let the blocks drop in comfortably when paired with a sheet of fiberglass. I'll vacuum bag the blocks in. I'll also just use carbon on the back half of the board (too much weight to go full in a situation where I'm not sure of the payback in stiffness/weight - or maybe I'll go full carbon and vac bag it so I can see the difference in weight driven by the process). Maybe another two weeks before this hits the water.