How Much Vacuum??

Check your lifter preload!

Or get a second vacuum gauge; 6" is not right and would/should be giving you tuning fits. And 16* initial should be plenty. Mine runs fine down to 12* and even lower.
Yeah guys crank 'em up to 25 all the time, and if you have an automatic with a 2200plus stall, that works.
But it don't work so good in a stick-car;on account of a stick car typically idles at 6mph@750rpm, and there is no TC to absorb the power pulses, so they go straight into the rear springs.And then the car starts to do this little annoying bunny-hopping, and in a bit more time, the entire chassis gets involved.But by backing off the initial timing, the pulses are weaker, less violent. And once you get a good tune into it, you find out that just because she likes 25*, she can be happy with 14* or even less. Mine "likes" 7*@600; the bunny-hopping being all but gone. I suppose I could put some SS springs back there, but then the pulsing would go into the chassis and tires. Besides, I don't want those springs.
Yeah so,back to the top^

I'm not sure how the lifter preload would've changed. I set each at 1/2" turn past zero lash and the motor hardly has any miles on it. Doesn't pop or make any unusual rattles either. I've tried initial timing from 10 all the way to 25. It definitely runs and starts better with more timing. I guess I can bump it back down to 15* and go from there.

What PCV valve is in there?

Ya know, I'm not even sure. It was on the 340 when I got it. I'm assuming it's stock, but obviously I could be wrong. Should I try disconnecting it?