I would figure it's an almost complete lack of ground if you're getting voltage at the end of the new cable you ran to the distributor, which should be almost impossible, because there should be enough metal to metal in the engine bay to at least get an arc to the block when you tested it. Although I suppose there isn't supposed to be. I'm hoping we can assume you aren't plugging into the tach terminal. I guess it could be something else weird, like your distributor isn't engaged on the cam, or something inside these units isn't assembled correctly. Rotor, coil. Weird.
For the ground, at least in the service manuals, there is 1 cable off the back accessory hole of each head to the firewall, and then the negative terminal on the battery goes right to the head. And on that spot where the negative battery cable attaches to the head, I ran another to the frame, on the bolt for the engine tower.
Here's a good post about ground strap location:
http://67-72chevytrucks.com/vboard/s...d.php?t=422234