Marine engines are just converted car engines. There are numerous heads that fit the 3.0. Suggest you find the casing number and pick up a similar head off craigslist or ebay from a car. That way you are assured to have the same combustion. Chamber, valve, and bolt pattern as the original and will be a direct fit? If the head you have has a different casting number anyone of those items may be different. You can crosscheck casting numbers across both to see if they will interchange.
Ayuh,.... Except the 3.0l has Never been used in any car,...
LOL, correct you are! Everyone I talk to mistakes it for being related to the 2.5l (151cid) iron duke which it has nothing to do with.
You may however find a 3.0 in a forklift, irrigation pump or ski lift.