Abstract: A method and a conversion kit are provided for assembling marine propulsion systems such as those comprised of a Mercruiser Alpha One® type sterndrive unit, an OMC Cobra® type inboard engine and a transom assembly, and attaching the propulsion systems to marine vessels. The method and the kit use commercially available parts and minimal special hardware. The technique involves a modified transom assembly comprised of an inner transom plate, a gimbal housing, a gimbal ring equipped with machined custom-made hinge pins, two trim cylinders with corresponding trim pins, and a bell housing with specially designed bellows and adaptor nipple. The invention is also applicable to the assembling of other marine propulsion systems that are comprised of equivalent combinations of similar sterndrive units attached to other similar inboard engines by means of transom assemblies.