83 slant six Canadian

I have a canadian 83 slant six van. My thermostat casing broke when we were screwing it down after replacing the thermostat. When I tried to get a new part, the holes for the screw holes on the new casing were different sizes. We drilled them (per the parts store) but now water leaks where we drilled the bigger holes. Are these differences in parts specifically because this van is canadian? Or is this part just not made anymore to specifically fit my van? Should I try to find another candian one at a salvage, or is this unnecessary?

I'd try a different parts store!!

Wow, you obviously drilled too far and got into the water jacket. Don't you know NOT to listen to parts people? They sell parts, not work on engines. About the only thing you can do now as a temporary repair to stop the leak is remove the hold down bolts, clean the threads, and sqeeze a bunch of RED RTV onto the bolt threads and area between the bottom of the casing and the mating surface. Let it set up for at least a few hours, but overnight for a proper cure to seal that leak.