Your hopes are a little high on your expected RPM with the HP you are planning to run. With 350 HP, you will most likely turning a "B" at about 4500 / 4600 RPM. This will obviously depend on your altitude and pump condition. I have 2 small blocks currently. One is a ZZ4 that's was mildly ported and dynoed at 405 HP. It turns a "B" impeller at 3400 ft @ 4700 RPM on a good day. The 434ci x 560 HP (dynoed) turns a "B" at 5400/5500 RPM again at 3400 ft.