The 391 FT was also used as a service replacement for the 428 because it could be bored to 4.13, as it was cast with thicker walls. Sometimes recognized by a reverse "105" cast into the front of the block just below the driver side head. I had one of these in my 1974 F100.

As for "And you have a race car of some sorts that you are going to put those heads on? "

You don't need a 'race car' to put these heads on. My 1971 Torino GT came stock with the 4V quench heads (M code) 11:1 compression, and went like hell. No, I wouldn't have gone down to a 2V on that car. That thing screamed lke a banshee in stock form.