Back to square one I think..
With ign timing at 10 deg, it will start cold, pull 15" vacuum but only for a minute or two. Seems like once it gets warm, it starts idling rough, vacuum drops, and it dies. No choke connected so its not that.

So I advanced it back up to the 30-40 deg timing range again where it will run and keep a constant vacuum. Adjusted the rockers by backing them off till they rattled, then tightening slightly.

I watched vacuum and didn't see any affect to adjusting the rockers. When I got them all, I set ign timing back to 10, and I'm at the same point as before..rough idle, low vacuum, misfire and backfire.

Adjusting the rockers cold, engine off, I was able to get all of them to 'bleed back' to zero on the dial indicator after turning 1/2 turn past zero lash EXCEPT #8 exhaust. I can't push the pushrod down by hand to compress the lifter and feel it spring back up like the others. So that one is bad but the rest seem OK.