Small Block Cooling Passages

I'm replacing the leaking intake manifold gasket and have some questions about the coolant passages.
The motor is a 1970 ish, 350. Heads look like they are iron. Intake manifold is an Edelbrok Performer rpm. The vehicle is a 71 C20 truck.

I see 6 coolant passages in the heads. Three per head. Looking at the heads:

The back 2 passages are blocked by the intake manifold which doesn't have matching passages.

The middle 2 passages are dry and carbon encrusted. The intake manifold does have matching ports for these passages.

The front 2 passages are wet with coolant. They do connect to ports in the intake manifold. The front intake manifold ports appear to be connected together and do not appear to have passages to the rear part of the manifold. The front passage has a heater hose port and the thermostat housing.

Is this normal? Why would the center ports be carbon encrusted?

the center ports that line up with carb location were for exhaust heat when cold to warm up the carb/manifold--worked by a heat riser fitting between exhaust manifold and pipe to make exhaust cross over under carb until engine heated enough to allow it to open----yes i know---dynosaur days

Thanks you.

But Dave, you didnt ask why the two back ones are not used......
it's another common question.


mike in tucson