I filled my 400 two inches from the block deck. I run over 500 hp n/a plus a 250 shot and the engine runs cool. I have an engine oil cooler as well as a tranny cooler running through the rad. I run dual 14 inch electric fans. You might want to consider filling the block 3.75 inches from the top, seeing as that is your stroke. In this way the entire travelled cylinder gets cooling. I think the depth of the water jacket is almost six inches.

An issue of greater concern for me is the routing of the engine coolant. Edelbrok has a wonderful intake which allows for the coolant to travel past all the cylinders, then exit upward and return back to the thermostat housing via external hoses. I partially block the front two ports so that I only have one eighth size return holes which forces the majority of the coolant to the rear before coming back to the rad.

Having said that, running 375 hp is far from dangerous. It wouldn’t need the filler.