Once again the "reverse direction" 4 bolt water pump rumor raises its ugly head. There is no such thing even though a number of parts houses have them listed. 55-59 GMC (Pontiac) V8 engines used exactly the same water pumps as like year Pontiac V8s.
I've used pumps with cast and well as stamped impellers to successfully cool my '57 GMC (Pontiac) 347" V8 in the Houston summers in stop and go rush hour traffic with the A/C on.
Look at the shape of the cavity in the timing cover that the pump installs in. It dictates what direction the pump must spin.
Previous rants:
http://forums.maxperformanceinc.com/...d.php?t=634794
http://forums.maxperformanceinc.com/...d.php?t=593938