Converting automotive engines for marine use has never worked they are just to different. I sudjest a 6cy Yanmar in the 250hp-300hp range with a Volvo Ocean series outdrive, their plastic so no corrosion. This sounds like a $20k project to go diesel, you will at least need new tanks and fuel lines, if your boat is an inboard, a new shaft might be needed, a new prop is a must and with the larger shaft a new stuffing box and larger strut bushing, or a new strut. Also the exhaust may have to be made larger. It might be better to just buy a new ford long block. Although depending on the condition of your boat it could make sense($$$) to repower with a diesel. Diesel boats are hard to touch under $80K and they will have old 1,000+ hour diesel's. You could have a nice new diesel in your boat for $20k-$25k, if you like your current boat this could be a good way to go. Also if you use your boat a couple hundred hours a year, a diesel could pay for it's self in the long run. Not to mention the resale value. Talk to someone who has done this before to similer boats, that way you will get first hand knowledge and experiance.

btw stay away from the Mercruiser diesels they are having huge problems. Volvo's are a pita to get parts for.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200