440 six pack high idle

Well, I started looking at the basics, beginning with the float bowls just to make sure I'm starting at square one. The outboards seemed to be adjusted properly, but the primary was bottomed out - empty. After some trial and error, I was able to get it set correctly. Reset the idle mixture screws to 1.5 turns out. I tried to visualize the throttle plates, and from what I could see they all looked closed.

I gave the engine a little more timing advance, initial is now at 15 - seems much happier, although the idle is high. With a vacuum gauge connected to manifold vacuum, I took a reading of 17 mmHg. Couldn't seem to get any effect on vacuum by adjusting the idle mixture screws, so I moved on to the idle speed screw. I tried lowering the RPM by adjusting it, but the engine would slowly sound like it was being choked out at anything below 15-16 mmHg, and then die.

The high RPM issue pretty much only surfaces after the engine is at operating temp, so I'm trying to think of what it could be that changes or opens as the engine warms up. I think today I'm going to look everywhere I can to make sure there's not a vacuum leak.