When I cranked my 351 for the first time, I had what sounded like a scraping sound. What I found was that I had tightened the lower radiator hose clamp in a position that saused the screw to reach out and just enter the path of the fan. The tip of the screw was barely contacting each fan blade and resulted in an awful sound. I would just check all of the external moving parts to be sure there is no interference. This is not always an easy thing to do with the engine off. You may not want to do this with it running, as you may end up missing a few digits. Look for shiny metal on the tips of brackets, bolts, or anything near moving parts. I was lucky enough to spot shiny metal on the fan tips. A flash light may help with your search.
Good luck
ssscode
Good luck
ssscode