Since you have the pump for a guide you could use an end mill. It doesn't make any difference where the old hole is located, it will bore straight in without walking. I'd try a 5/16 wrapped with shim stock to hold it centered in the pump hole. This should let you collapse and remove any remaining bolt. If not use a new sharp high quality 3/8-16 tap and thread into the hole cutting away the old bolt as you make new threads. If the hole is so far off to the side that the 5/16 end mill doesn't make a round hole a tap will hang where the notch is. This means one goes with with a 3/8 end mill, remove pump, drill 3/8 hole with drill supplied with heli-coil kit, thread with heli-coil tap, insert coil, and it will then be the strongest thread in the block. You can get end mills at machine tool supply places and will probably need a 1/2" drill motor to chuck them up. Regular ones are under $20 and long ones are still reasonable. I save ones from the Brigeport that have only one flute chipped just for jobs like this.
