Only cheap HEI works

While 15 IS overcharging, it is not serious, and would not cause some ignition problem. Overcharging only has so many causes, in no particular order

1...Usually, voltage drop in the harness, including the ground path between regulator and battery NEG

2...Regulator going bad. Make sure engine is warmed and battery is "normalized" before making a decision on voltage. This is because some/ most regulators are temperature compensated, or at the very least, are sensitive to temperature. They normally run high when cold

3.....Certain battery problems can cause overcharging. Simple, usually, to swap in a known good battery

4...Other wiring problems

HOW TO CHECK for voltage drop problems

1....Hot side ....Turn key to "run" engine stopped. Stab one probe of your meter into the top of the battery POS post, and connect the remaining probe to the "igntion run" line as close to the VR as you can. You should read a very low voltage, the lower the better. More than about .3V check to find out why. Generally, bulkhead connector terminals, igntion switch connector / and switch itself, or bad connections at the ammeter.

2....Ground path. Run this two ways, with engine running "fast" to simulate low/ med speed cruise. First check with all acccessories off, and gain, with heater, lights, etc on. Stab one probe into the top of battery NEG post, and the other onto the VR mounting flange. You should read very little and zero is perfect

ANY DROP in the above two tests is ADDED to the VR set point, IE if the VR is "correct" at about 14v, and there is a 1 volt drop, the battery will "see" 15V