Blowby, can someone explain...

I had the exact same problem. My car ran fine w/11 psi boost. i actually ran it at the track a couple times with the problem. couldnt tell any power loss, But the breather was puffing smoke. for a month or so i just put socks over it and changed them every couple days. I did a compression test and found 2 cylinders low on compression. Then i did a leakdown test and found the same two leaking into the crank. Pulled the engine apart and found small cracks between the rings on the piston. When I looked closer, the crack had actually broke in two places and an entire chunk of the piston pulled off. I would say if this has slowly progressed over time it could be worn rings, but if it happend after a couple days of hard driving it could be the same problem i had. Good luck

The block was fine, the cylinders were still perfect. I just needed to replace the damaged pistons. The cylinders kept the damaged pistons intact by keeping the crack pressed together. I decided to swap in a 351 instead of putting the 308 back in. I would do a compression test if I were you. If you find a couple very low cylinders that will be a red flag the problem is more than just worn rings.