Drill out the oversized hole for a 3/4 inch bolt. Drill and tap the center of a 3/4 inch bolt with a 7/16 tap. You need a lathe to keep it centered. Cut off the threaded part of the 3/4 bolt and scew it in the hole with locktite. You should run a tap in all the bolt holes in the block before assembly. If you find any more helicoils you should scrap the block. Whoever put them in didn't do it right.