So I may be jumping the gun, but I was planning on replacing the engine soon anyway so I might as well prepare in case it is indeed cracked. What would be a better option, refurbed F 351 w/ 8 yr warranty for $1900. Or a used engine from from junk bronco (F 351) w/ 100k miles for $1000. Also, are there any "auto to marine kits" or could anyone advise which parts need to be changed?

Go rebuild and I think freeze out plugs are brass vs. metal is the main thing, I think.

Marine engines often have totally different cam's than you would find in a Bronco. In the GM 350's they use the heads and cams from heavy duty truck applications to develop more low end torque. I would go with rebuilt so that I would be sure of what I am getting

Cams and freeze plugs are about the only differences. Now if you need to replace more than the block, just about everything else is different. Rapido Marine is pretty well respected over at iBoats (never used them myself). They can ship you a 351 marine long block for about $1500 last I checked with no core charge. If I were going rebuild, that's who I'd go with. That would be a lot better solution than marinizing an automotive engine.

yeah, I've read a lot about Rapido marine. They seem pretty good. If I go rebuild, it will be Rapido Marine or USEngine.com. Rapido Marine is a little cheaper, but US Engine is local within my state (WA) plus they give the 8 year warranty...which is unheard of. Too good to be true maybe?