What to look for in U-pull Magnum?

I pulled 4 darn mag heads at a yard to find them visually cracked, then found 2 that were not, took them to the shop to find they magnafluxed as cracked. Before that, had a set of 308 heads that a bad eng shop did a valve job to, took them to a different guy who started cutting the seats and found both to be cracked in multiple places. I personally don't see the point of a valve job on a cracked head, unless it's going in a stock, low budget farm truck. If I had saved all that time and money on getting cracked heads removed, inspected, and even rebuilt once, I could have almost bought a new set of eddys and been done with it all months ahead of time.

Just my 2 cents.. junkyard heads aren't worth it.