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

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!