360 cam selection

I'm kind of in the same situation, and I'm a little confused about the part where it comes to shortening the valve guides...on the Hughes Engines website they say anything over .450 lift could have clearance issues, then on here on another thread someone mentioned that you're probably fine till .500. Any consensus on when the guides have to be cut down? I'd rather just drop in a cam...right now this thing just falls on its face past 4,500rpm. (71 Dart, 360/727/8.75, 2,300 stall).