Replace the thru-hull fittings because you will have to cut the old ones or new ones to the proper length in order for the flange on the seacock to properly mate with the hull. Cutting them accurately in place will be difficult at best. You will need to factor in the thickness of the hull, the backing plate and the thread depth inside the seacock, then make your cut with the thru-hull in a vise using a sawzall. I run the backing nut (upside down) that comes with the thru-hull fitting down the threads to the cut line and use it as a guide for the saw blade and then when removed it will chase the threads. Additionally, when buying your seacocks ensure they are FULL-FLOW.

Brian Gaynier