A nice litte trick to use. Since, by the pics, it looks like you have several threads down into the hole before you come to the broken bolt. Take a short, same size bolt (looks like a 3/8 from the pics) and put it into a vise in your drill press. Center punch the bolt in the center, then drill a small (1/8th) hole all the way thru the bolt. Thread the bolt into your engine and using the same drill, drill your first hole into the broken off bolt. The pilot hole will keep your drill bit from walking and you'll get a nice hole down the center of the broken off bolt. Then just unbolt the pilot hole bolt, and start working on the broken off one, either with a left hand drill or using ez-outs or whatever. The real trick to getting the bolt out without messing up the threads in your block is to make that first hole as nearly centered in the broken off bolt as possible.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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