New manifold, lots of holes??

[Jeff Chambers]

A) For heater core return (supply would come directly off the water pump).
B) Best place for water temp sensor.
C) Either air charge temperature sensor, vacuum guage, or as a supply for your vacuum tree. This is the only place to draw manifold vacuum. Don't see how you could do air/fuel ratio monitoring since the mixture hasn't been burned yet.
D) Plug it. Don't take your water temp reading here because it will read quite a bit from the front port.