90% of our heads probably have the valve to valve crack. It took me several sets of heads to find good ones. Will the crack spread or open up into the water jacket..... Who knows. Maybe have your local shop check them. Maybe you can run them another 100k and be fine.
I personally did not feel comfortable with cracks worried they may open up into the water jacket or cause a hot spot on the edge and get preignition. Running boost I wanted to be more conservative personally but there are many stories of guys on here running cracked heads without problems. I would evaluate how big and deep the crack is. What is your tolerance to risk, reliability, how much boost, and how long will you have the car. Many people run cracked heads and are fine.
