The block inlet, I thought was the water return after oil cooler, its been cut off.
Now I've got it apart, water seems to go, pump, oilcooler in, oil cooler out, heater in, heater out, then back around to pipes/fittings on RH side of runners/manifold at top of engine. And your right, there is a small metal tube runs from back of valley to the return pipe. In this cct there were two (2) 90° kinks in the hoses - flow would have been seriously impeded
I wonder if Corvette Qld would have still lying around any of the tubes, valves etc after their conversions, 5/8"ID rubber are bulky and dont bend too well.
There is also no heater water cut off, off course if there had been one in this cct, it would also cut off the oil cooler, and possibly negate your air bleed.
I know I'm being pedantic, I just want it as original as possible, a heater here in Oz is overkill, but when A/c running, is only way on flooded accumulator system to controll the outlet air temp.