Marine engines are basicallly the same as car engines, with a little changes in the intakes and that. From what I understand. Alot of boat, the bigger ones have 327 and 350s, so there should be a few with the 216 in them. Its a good way to find a cheap engine that needs a rebuild. Rebuild it and change out what needs to be done to put it in a car. Basically all you need it the block, as everything else should be replaced in a rebuild anyways. Any used engine you find will more than likely need to be rebuilt.
Oh also the crank should be the same in a car and marine engine. so that could be reused if in good shape.