If you have not seen a crack in a magnum head you have not looked close enough. It is VERY hard to find a set that is not cracked, I have pulled tons of those motors apart and have found mabey 6 heads that I could not find a crack between valve seats, probably %90 of them are cracked, I have even seen NOS castings that were cracked. Now you got to think to yourself, do you see magnum powered trucks blowing steam out the tail pipe all the time? Neither do I. I have pulled heads off motors that ran great with low miles and no sludge(evidance of overheating) and found cracks in them If you do find cracks get them preasure tested, they are probably fine, I have never seen a set that leaked from the crack. The cracks are not huge, you need the valve seat very clean to see them, I use a wire brush in a drill, polish the crap out of the seat and get a bright light on them and you will see the cracks

On the other hand if you don't clean them up and find the cracks you can just pretend they are not there and live happily ever after.