Broken bolt in front of crankshaft

[ElCoSix]

Aargh!
I have a 67 El Camino with the 250 inline six that I'm restoring. I'm going to use the six for now - and maybe forever. I'm trying to get a broken bolt out of the crankshaft without tearing it apart. There's no radiator or even core support yet, so I can get right at it. I was thinking that if I had a new bolt, I might be able to get a machine shop to drill right through the center so it would make a nice guide to drill in the very center of the broken bolt. Then I could start the bolt in and drill a pilot hole right where it's supposed to be. That way I could try a reverse direction drill bit or some kind of extractor. Does anyone know the thread size and pitch of the bolt or where I can get that information? Or better yet, does anyone have one? Or any other ideas for getting the broken bolt out?
Thanks a lot,
Scott