they are a few and far between but that arent all that rare. i know where 2 are sittin right now.as far as the 351c. my mom had one in a 73 mach 1 back yrs ago. it was a dog.she sold it to a girl who had someone build it up some and it run good then. first thing they did was new cam, intake and a holley 600.then laster they did change heads but i dont remember what they used.