[bulletbob]

On any vehicle , with a prop or wheels, almost never can you even come close to recouping the cost of an engine replacement.. Thats reserved for something you love and wish to keep forever, not caring about if its "worth it" or not...
I went online and looked around and the 454 marine is available at several places.. A place called US Marine offered a series of years of rebuilt 454 engines for just 2 grand each, to about 4 K,with a 7 year/500 hour warrantee...
They might be imported however, do your research.
Complete crated new 454 marine engines were around $ 6000-7000 from other dealers..

Google 454 Marine engines, and lots of stuff pops up.
You can also have your engines rebuilt... Seems to me, 2 rebuilts w/new manifolds, etc, keeping pans, and whatever other parts are solid and rust free should be no more than about 12-14 K or less for the parts... I was in the auto business for decades, and bought lots of engines.. Rebuilts these days are VERY good... I would shop around, get 2 rebuilt 454 marine engines with solid warrantees, get whatever peripheral parts the installer recommends, and just get the right marine mechanic to do the work.. It would seem it could be done for well within what you want to spend. Shop around .. a LOT. Some places charge double what others do, just because they can get it. bob