My 2 cents
I had a guy come to me about 4 wks ago with a 68 Chevelle ss396 that had been running bad (he complained it wouldn't spin the tires, BTW the tires are 6 inches wide). He told me that he had 2 cams go bad in the last 2 years (both comp cams). I took it for a ride and it wouldn't go past 5K RPM and it "popped". So I put another cam in it (Lunati VooDoo) and broke it in VERY carefully. In resetting the pre-load on the rockers I noticed that the roller tips on the intake valves were about half off the valve stem. So the more I observed the more I thought that the guides were off. So I took one guide off and matched it to stock BBC guide and sure enough it was off by quite a bit. So I changed them all. Now if you think about it the guides being off could have put enough bind in the pushrod/ lifter to not allow them to turn, I dunno. Anyway, so far it's OK..