Grind or file the front of the block flat. Use a straightedge across the pump mounting area, and a feeler gauge or a depth micrometer to get a measurement, and keep filing until it's exact. Then fabricate the spacers so the gap is eliminated. Stuff like this was "auto mechanics 101", that every journeyman mechanic was expected to know a few years back. These days, if something isn't a perfect fit right out of the box, nobody is interested in doing anything that involves thinking, or working!
Jerry
I grew up helping dad as a mechanic, and later got into doing fab work, and for the last 10 years I have been a machinist. Today I work with fabricators and mechanics every week and many of their ideas make the machinist inside me cringe. Everyday we assemble and rebuild equipment thats wore out or was not made quite to spec but we always make it work. The straight edge and feeler gauge just doesn't work for me. If I cant sweep an indicator across the face I cannot say its exactly flat or parallel. The best way to do this would be to set it up in a milling machine and cut a counterbore square to the surface just deep enough to clean up and flatten the block. And If I really want to complicate things I could make a new plate thick enough to machine some raised bosses on the backside and it would almost look like this water pump belongs on this engine. But if I wanted to go to that extent I would probably just build a new pump from scratch that fit the engine holes but just had a raised impeller shaft. I am just in a weird position with this project. I need it done good enough that it works and is reliable but I need it done somewhat cheap and fast so I can use it to build my dream house in the spring. I refuse to buy a new plastic truck for that use. Once its done I have every intention of completely restoring it back to like new original perfect condition. While building it this first time I am making notes of everything that needs to be further addressed when I do a frame off restoration in a year or two. Its not that I dont want to think or work I just need the most sensible way to get this truck running for the spring. Maybe I will look into renting a magnetic drill and using it to counterbore the block with about a 3/4 endmill. I could also use it to drill and tap the new holes square. It probably wouldn't be perfect but just good enough to satisfy my OCD machinist mentality.
