i'm not a ford guy, but i know some of the older chevy cylinder heads do not have the mounting locations for the marine accessory brackets. Double check your heads to make sure you can bolt all of your brackets onto the old mustang motor/cylinder heads. Also verify that the exhaust manifolds have the same bolt pattern and are compatible. Chris is correct, the camshaft will not be optimized for marine use, but should work. Depending on what intake manifold you have, it may be the same case as the camshaft.
I'd verify the donor motor is good with a compression test and leak down test before i even considered it. Just because it was rebuilt a year ago, doesn't mean its a healthy engine. A lot can happen in a year, and "rebuilt" can mean a lot of different things to different people. Why is the mustang owner getting rid of an engine he just rebuilt last year?
1988 Supra Sun Sport 454 PCM
1972 Olds Cutlass 455
1998 BMW 750il
1996 BMW M3 coupe
1995 Toyota Tacoma 4x4