Mine started seeping at about 80000 miles. I've drag raced the truck for 3 years. I'm in the process of swapping in an LMM shortblock and there is a noticable difference between the old gaskets and the new.
My truck had B's on both heads. I put in C's because they where available. Head gaskets are getting to be like the new LB7 cancer. Seems like every day somebody loses one.