The stock 1/4" bolts will not take the additional stress of the new rudders over time. Replace with 5/16", usually not a problem to carefully drill and tap. The al backing is pretty thick if it is still anchored to the transom. DO NOT Peel off deck. With patience and ingenuity you can work from the gap at the bottom of the bulkhead. If you want to go faster or through-bolt, holesaw a handhole through the bulkhead.