I can't say I've "built" a 4.3, but my father and I dropped a Comp Cams Magnum 270HR cam into our '03 Bayliner Capri with the Merc 4.3, then tossed a Paxton Novi1000 onto it in a blow-thru setup with the Merc factory 2-barrel, rejetted of course. If I remember right it had Vortec heads on it...