I'm planning on running a Comp Cams 507/510 lift that I can't remember the duration of right now, I think 24X. I'm not real sure what the max lift is for X heads, but I think I'm alright with this cam. Not sure tho.
In stock configuration you'll be lucky if X heads can accept anything over .500 lift. The retainer usually hits the valve guide. You need to have it machined down then you can run a pretty decent size cam.
Just for reference I tested 2 sets of stock heads to see how much lift they could take before retainer/guide interference became a problem. One set went to .510 and the other set only went to .475 lift before it was a problem.
