If it's the coil getting heat soaked and causing issues, run it until it acts up again, then cool the coil rapidly (big chunk of ice held on the coil side for 5+- min, then see if it runs good again for a period of time until the coil heats up again. You could also test the coil with an ohm meter, cold, then when the engine is having problem. (I don't have the resistance measurements off hand...)
Being a 72', if you haven't changed points and condenser, I'd do that also.
If you're going to dump a bunch of money into the ignition, look at Delco EST for the 140HP/Merceuiser 3.0L
