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Actually, thats not right. A big block has two waterpump holes at each side of the block(r&l). Water does flow into the block from the lower hole, around the cylinders,(and normally thru holes in the deck to the heads in small quantities) to the back of the block, where it enters the head and than flows back to the front water passage, into the front of the block. The front water passage in the block is separate/divided from the inlet waterflow. Remember, a big block has the t-stat housing on the pump, not the intake.
The picure shows the upper small holes in the deck, but on a siamese block, air gets trapped in the hourglass shape on the lower deck because the air cant escape between the cylinder bores. Your gaskets will have holes in them, and my guess would be that your heads have holes there also. It may be as simple as drilling a few 1/4"holes in the deck to get rid of this problem.
The timing you report seems awfully low. Do you have a real small cam?



Your correct on the 4 holes in the block...my error