I have a 1969 396 block at the machine shop. They recommend that the the block is modified to include water passages on the block to match the heads. Is additional water passages common modification? My block is a 325 hp block, but did not come with my car from the factory.

Usually we restrict the block deck surface to promote coolant flow around as much of the cylinder as possible, at least in the SBC. The gasket is a restictor in itself. I would ask for a good explanation as to why your machinist thinks this modification is needed.

Randy,

It isn't a bad idea but not really needed on an engine like yours. It is pretty simple, just drill a couple of holes, and you have to use the new style head gasket to match. It allows more water to circulate up through the exhaust side of the head. I think GM started doing it in the 80's sometime.

Jim, this is a different mod than on the small blocks.

Bill and Jim, thank you for the reply. I not building my cars to be racing on the tracks. My engine builder recommended the additional holes as GM had added the cooling passages in the later engine blocks. My goal is to build a nice driver car, therefore I will not modify the block.

Bill is this a needed mod if you are using the early head, block and gasket? It seems to me that was the case and everything changed with the redesign in 80, but only if using later heads on the early block.
Jim