There is very little to weld to in that corner, 3/16" maybe from front to threaded hole, and then you need to keep the hole round and threads usable. You plan on welding the hole shut then drilling and threading, or you going to try and weld the crack and try to cut new threads in half the hole? You would be better off, cutting the corner off and welding a stud to the block and building up enough to face off for the water pump surface. The threaded hole is already compromised, it's no longer round.
I had one of these blocks exactly like this one. Everyone I spoke with said the whole block would need heated and the hole welded shut completely. Then the machine shop would rebore the crank holes, cam holes, cylinders, recut the deck, drill and tap the head bolt, and surface the water pump hole. I bought a whole running engine for less than the welder wanted.
