FYI,
I purchased a water pump from Salvo's for my 2000 Chevrolet Silverado 1500 with the 5.3 engine. The number on the box is 18-1652/20333/WP5107/WP5104. The bore for the thermostat housing is too small. I had to use a die grinder to open it up for the thermostat to fit. Now I have a custom water pump.