JR North wrote:

2. Vacuum leak-definately. Big one, too. I vote for brake booster.
HVAC -not enough to materially affect idle. Intake manifolds replaced;
unlikely but possible leaks bottom side of intake runners at head
ports.

When the heads were rebuilt, did they surface them?
A deep cut or several light cuts can leave the manifold mismatched to
the vee formed by the heads. If you end up pulling the manifold, check
for port matching. Look at the gaskets you remove and try to determine
if they were sealing all around each port.