I think they have a 10cc larger intake chamber on them. They are 71-72 350 castings like you said, passenger and truck heads. The rest you probably know, 76cc chambers, 1.94/1.50 or 2.02/1.60 valves. They should flow around 215-225 cfm intake @ .500. Intake runner should be somewhere around 165cc, exhaust around 65cc. There was a time that I wanted a pair, but I have a pair of stock 487's that came on the 72 if I really want a pair. I almost bought one of the 487X's. I'm thinking it had a crack in the chamber, and the guy didn't want to part with it.