i'm experiencing a similar problem, with my standard '41 setup. and have performed many of the tasks you have performed, trying and failing, to fix it. new distributor, points, condenser, plugs, wires, carb, fuel pump, etc., etc. starts readily, runs great at idle, and for about ten minutes out on the road at speed, and then begins the inevitable misfiring. progressing to the point, that i'm very, VERY relieved that i made it home without a calling a tow truck.
lately, i've been contemplating running a jumper directly from the battery to the coil in hopes of bypassing complications involving unwanted or possibly detrimental resistances. when i do it (and it could be a while, as i'm working on the parking brake system), i'll report my findings...
i'm not all that concerned about burning up the coil, as i have two new ones, that i assume are working properly, and a third that came with the car, that i have no real reason to think is not.