1969 dart loses spark once it warms up

What do you mean "I have spark from the coil?" Under what conditions?

If you have a good hot spark from the coil WHILE CRANKING WITH THE KEY then it should run, and MAYBE you have a fuel problem and not an ignition problem

Also mounting coils vertical "may be bullshit" because nowadays many coils are epoxy, not oil filled. Ford coils were mounted horizontally for years. So were B/RB Mopars