Had somewhat of a similar problem with the 493 race motor. Ran very hot (250 degrees), cylinder head temp was 100 degrees different (hotter on passenger side). Pulled the water pump and blocked the bypass below the thermostat housing. Adapted a garden hose fitting to the temp sender on the waterpump housing, removed the top rad hose and clamped it to block the flow through the radiator. Turned on the hose and noted the water flow from both sides of the waterpump housing. Our passenger side was blocked; very little flow compared to the driver's side. Turned out the block was partially filled and the back two cylinders had residue between them preventing water from exiting the block.

Coolant flow on a big block is isolated on each side; there is no cross-over. With the garden hose attachment, water flows into the top hole of the block (from waterpump housing), up into the front of the head to the back of the head, into the block and foward to exit the lower hole in the block. There is a passage on each side of the front section of the waterpump housing where the water will flow out and it should be the same for both sides.