Help an old man tune a new to me 340

There is NOTHING wrong with the Edel carbs that could cause your problem. The cam must be fairly mild to idle with 15" of vac & only drop 100 rpm going into gear with so little initial timing. With more idle timing, the rpm drop will be even less, maybe only 50 rpm.
The plugs are too cold, could be your problem. Try some NGK 5 heat range. With cold plugs, the deposits do not burn off because the plug electrode never gets hot enough to do so. Spark energy is bled off by the deposits causing a misfire.....
The fact that the carb idles & makes 15" of vac tells YOU it is reasonably well adjusted.
* Warning sign*. The change from ported to man vac adv should have changed the engine sound & rpm. No change could mean the vac adv diaphragm is leaking or the carb's T slots are not in the correct place when idling.
Try this simple test:
- loosen met rod piston covers, swivel slightly so pistons are in view. Tighten covers.
- adjust initial timing to about 20*, not critical.
- warm up & put in gear. Pistons should go down & not move. If they move, you need softer springs.