That sounds like one of those deals he should probably pay you to take it. Sure, it was a great engine once, but years in the barn after being in the 'pullin' truck' have reduced it value to just above scrap. If money weren't an issue, I'd say cut him the grand before you sank 4 grand into it. If you can't do that monetarily, I'd find some other option.
And you may have to alter your pony significantly to even get it to fit. (BB shock towers, right?) I'm not an expert on BBF, but I think that's how it works.