He might be looking up the wrong end of the trans ... it's usually the input shaft that's got the ring grooves on it.
One more thought on the 1970 455 HO. It was a one-year-only motor, so finding the heads is a little tough. It was also not put into the Firebird, to the best of my knowledge. I wish I had the Firebird supplement to my 1970 manual.
I recall that there was a Formula 455 available in 1971 and 1972, but I am not sure if it was available in 1970. Also, the T/A in 1970 only came with a Ram Air 3 or Ram Air 4 motor (both 400 CID) not a 455 HO. That motor was the standard T/A engine in 1971 and 1972, and those 455 HO motors were the round-port type.
That would make the 455 HO from 1970 a GTO and GP only motor, which makes it harder to come by, but - as Tom said - not necessarilly the most sought after.
