Vacuum leak/idle issue

[butler57]

So...here's the backstory. Truck was running fairly well and idling fine. Took it out at highway speed last week and it really started idling rough after that. It's a re-built stock 350, HEI,with a Holley 650 carb.
I suspected a vacuum leak so I've sprayed carb cleaner along every gasket and can't seem to find one. I've replaced all the vacuum hoses - no change. Turning the idle mixture screws makes no difference - can't get it to bog down. Also tried a piece of cardboard over the carb and can't choke it off at all. PCV appears to be working fine. Changed all the sparkplugs and checked all the plug wires too.
Even with the idle mixture screws all the way in it seems to be running really rich and it's really burning through fuel.
Replaced the fuel pump not too long ago. Doesn't appear to be a fuel problem as I see gas when I move the throttle (unless it's getting too much?).
I'm not a mechanic so would really appreciate where to go next?