Seems like the springs were 145# seat and 350+ at .500 and plenty of room to go .650 lift. I do.
I've put several 6500 rpm hits on them and everything is still in one piece.
Right now I'm only running a 525 lift with 1.6:1 roller rockers and I don't make any more power after 55 to 5800 RPMs.. I've spun it past 7500 but it doesn't do anything but wear the out motor faster...
I don't see myself putting any kind of hurt on these heads..