Bolt size Chevy 350 short water pump?

the bolts are 3/8-16 . as for the length, all that is necessary is about 3/8 of an inch of bolt sticking through the pump, and gasket.

a rule of thumb is the length of bolt you need to secure something ,is equal to the diameter of the bolt. that is besides the thickness of what ever you are securing , you should have a bolt at least the diameter of the bolt longer.

example- if the pump is 1 inch thick, your bolt should be at least 1 3/8 inch long.