Water pump bolts

Do not trust any measurement you are given. You have access to measure correctly.
That bolt goes through the water pump, all the way through the timing cover, and threads into the block itself, usually into the water jacket.
Measure that length, add thickness of your brackets that bolt passes through, and then add about 3/8” to 1/2” for the actual threaded portion. That will be your true bolt length needed. Size diameter is 3/8”.