Most likely a 427 center oiler with cross bolts. The heads and matching intake are pretty cool. High Risers were also called "tall and terrible" for the very large port size requiring special intake gaskets. NASCAR outlawed them in 1965, something about an unfair advantage. Ford's response was the "medium riser" which was "almost" as good, almost, not quite, nice try, but stills falls short of the "Tall and Terrible" category.
It's virtually certain that is indeed a 427block. The High Riser heads also ran BIG valves which would not clear the bore of a 428, 390, etc. They barely cleared the 427 bore.