HEI Conversion Done still not starting

"I now have spark" means there's probably nothing the matter with your ignition system. I'm going to guess you've had the distributor out at least once, so you've probably checked and approved the condition of its drive pinion (or at least you've verified that the distributor is in fact turning when the engine is cranked), right?

Next up on the list: ignition timing. Very easy to mess it up on the slant-6. See here.