Just throwing out some more first hand experience with those cracks.
I do quite a few 6.0/6.4 stud jobs on the said, every set ive taken off have cracks, some worse than others. Most time the shop I take them to install all pressed in seats to. Most times that cuts the cracks out.
So far I have had no issues with this probably 10 trucks so far, and the machinist does this to every 6.0/6.4 head he gets and said he has not had one come back.
Also another option is to have them pressure tested and just reuse them, ive done this on two trucks so far. The customer choose not to have the seats fixed. So we just cut them flat and reinstalled. No issues with those trucks either, at least not yet they both have about 20,000 on them now.