Stop in for a cup of coffee

So last night, I got the Polara to run for a long while, probably about 30 minutes or so. Everything was going good and got her timed in, water temp was good, holding steady at 180 degrees, oil pressure at idle was steady at 65 psi.
Then it all came crashing to a halt, she started smoking really bad, out the drivers side breather. Not out of the exhaust. Thinking maybe a valve seal let go?