You have your work cut out for you, I think several of the Mustang web sites have done the I6 to V8 swap, there's a lot more to it than just changing the motor. This swap would really make it go. First off the 351's have larger combustion chambers, so compression has decreased, unless he used higher compression pistons to compensate. I think the price depends on how good a job he did on the rebuild, a rebuild diy would probably cost between 400 and 600 unless you had to bore an engine and install new pistons, and not including the price of the motor. Sbf engines are plentiful and cheap, you might want to consider a 5.0 from a salvage yard and diy rebuild. I would look at the areas where the gaskets are and see if he used silicone adhesive, this might give an idea the quality of work he did. There are ways to cut costs on a rebuild, that I personally don't like. The other problem with this is you don't know how much wear in on the cylinder walls, if it has a lot of wear, even re-ringing it, it could have a lot of blow-by. You might ask him some questions, like what he replaced on the rebuild, then it depends on how truthful he is. Good Luck.
