Try a new coil.
This sounds almost exactly what happened to my ramcharger while on a camping trip a few weeks ago. Started it up in the morning, drove off, not a mile later it dies. Wouldn't start up. Sat for a while, starts up, runs like crap, suspect the jetting from when we leaned it out at the trailhead before climbing a couple thousand feet. Screw with the jetting for hours while it runs, drive off, dies, runs, dies. Messed with timing, fuel, threw on a new ecu, nothing. Finally smoked the coil so much I could hear it sizzling oil out of the posts. Limped into town, pouring water on the coil every mile or so to cool it down. Heat only increases the resistance inside the coil, making spark weaker. This would explain why it runs, dies, runs, dies. When you have that key to "on" that coil is getting 12 volts all day, only getting hotter til it can discharge that spark.
You can visually have spark when you test it but when you try and make it jump a gap with 170 psi of compression it takes a strong, healthy spark.
Roughly get the timing close like the boys here said and try a new coil. It's the only thing you haven't tried. Is it a 12 volt coil??
And 67Dart - I love that emergency HEI - Coulda used that a few weeks ago!! Always bringing a spare coil with me. Actually I'm going to build that little kit you have. Thanks for the idea
