I installed a Summit HEI in my '71. Car started fine 1st time; there was no spark second time. I have 11.9v with ignition on then it drops to 6v while cranking. Could this have damaged the coil and/or module? Do I need/where can I get a 12v source during cranking?

At least on a '69, there is a post on the fuse block that's marked IGN and a standard spade terminal fits. Run at least 12ga directly to the BATT terminal. Low voltage 'could' damage module. You'll just have to see.

Are you still runnung the resistor wire?