I had that happen to my 68 first time I installed a pertronix. Drove around fine, parked it, went in to a store, came out and it wouldn't start. Ended up towing it home. I found that the plate that is mounted inside the distributor with the pickup mounted on it, needs to be grounded really good. It's mounted with one little screw and if its not tightened really good, it will not start.
I tightened it up and haven't had an issue since (going on 6 years now)