C lectric and Action’s advice is spot on. I used the Groco flanged adapters and would just add that Groco also makes a composite backing plate that you can epoxy to the inner hull. The flange base bolts to the backing plate from the underside and makes an extremely strong connection without having to bore three bolt holes per through hull into your boat.
Follow Rod’s how-to on this—and on anything else boat-related—and you won’t go wrong.
https://marinehowto.com/replacing-th...-and-seacocks/