I thought the same thing, but have had 3 different pumps on this motor and I have good flow coming back into the top of the radiator from the heater core.
Each half of the engine (with its associated head) may have different coolant flow properties that tend to cause higher flow on one half vs the other.
A pump imbalance could also exist at each outlet of the pump, but after three pumps, the flow in halves of the engine seem suspect and not the pump(s).
The flow in each engine half could be readily measured with pump removed, engine off and thermostat removed of course, and
- a water source (garden hose),
- an adapter (garden hose to coolant inlet hole in block),
- a blocking plate (to block off the
opposite coolant inlet hole in block),
- a common water pressure gauge (to measure the pressure of the applied water, i.e., measured at the inlet to the block),
- a pail (to measure the outflow from the thermostat housing), and
- a stopwatch.