I think it is possible to get 320 from factory iron heads but you have to do a lot of work. Probably cost $5000 by the time you're done to get a good set of heads that would flow those numbers. Dwayne might know, he might have worked on a project like that before.

I haven't seen any of the current 906 heads used for Super Stock racing but I'd imagine they flow numbers like that. I bet they cost a ton of money and don't last long either.