I use an 8 blade pump with my 340
I don't use underdrive performance crank pulley from March. At first I did but was overheating, replaced it with a larger pulley and am cooling fine. The underdrive pulley will slow down the water pump to gain a couple of horsepower

also Flow Kooler has a pump with 16 blades
http://www.flowkooler.com/pump.php?pid=WP1700&pid0=TS370
never tried one, just a heads up