TN 250 6 engines and parts

[ElCoSix]

Hi,
I have a 67 El Camino with a 250 that I'm restoring. The bolt is broken in the front of the crankshaft. I know it wouldn't be the biggest sale you ever made, but do you have one of those bolts - or do you know the thread size and thread pitch? I'm trying to get the broken bolt out and running out of ideas, but if I can figure out the exact thread size and pitch maybe I can at least clean out some of the threads near the top of the hole with a thread chaser, and that would get me to the next step. (If I can get a bolt started and stable enough I'm thinking maybe a machine shop could drill down the center of the good bolt so I can use it as a drill guide. That way I can get a good, centered pilot hole drilled and maybe then have better luck using an extractor or even a reverse direction drill bit.
I'm really hoping I don't have to tear it apart to get this done.
Thanks a lot for any advice or help you can give me.
Scott