I finally figured out my mistake. SOME of these cars used a rubber gasket set into a groove. Others used a metal gasket that you need to put sealant on both sides and assemble. The metal gasket has a groove in it but once bolted down that groove will provide the seal as it flattens out. They provided both type gaskets. I only needed one. I did not have a groove built into the pump mounting surface so I needed the metal gasket. I saw a couple videos of others changing a water pump on a Mercedes and they had the groove for the rubber gasket. Interesting since these are Mercedes motors. I'm all assembled now. Just need to fill it up with coolant and hope I don't have any leaks. Fingers and legs crossed!

ONE MORE QUESTION........I drained the system today at the radiator petcock. I got about 7-1/2 quarts out. Do I need to worry about the coolant that in still in the engine block? The coolant I drained is blue but I'm putting Zerex G-05 back in. System holds 11.8 quarts...just short of 3 gallon so there may still be about 4-1/2 quarts still in there?