5/16-18 thread uses a letter F (.257 diameter) tap drill or ..
If you purchase real helicoil inserts , not the recoil brand, they are made from Stainless Steel...no worry of rusting.

 

I mis-spoke, the block will rust away where the heli-coil has been installed.
Jim

 

my dad showed me a trick of welding a nut to broken off bolts and presto it works great.
i am a diesel mechanic and i have used that trick tons of times and it has saved my butt alot

 

since all the water pump bolts are case harded bolts drilling will be a royal pain if you can at all. pull the pump use vise grips and a torch if you you have one.

 

Water pump bolts are SAE grade 5 at the most. They are not case hardened. After several years in a car, they tend to corrode and rust in place. The best option is to grind the exposed end flat, spray with penetrating fluid and wait overnight. Additional applications of penetrating flud can't hurt. Then mark a center on the end with a punch and drill a 1/16 inch hole as clost to the center of the bolt and as deep as possible, then work your way up to 1/8 inch. The heat generated by the drilling action may help loosen up the bolt. Then use a left hand 3/16 bit with a high speed drill in reverse. Pray. If that doesn't free the bolt, continue with right hand numerical bits up to the recomended size to tap the hole 5/16-18. I wouldn't use any flame or welding solutions near the block while it's still in the car and assembled.
Jim