Testing alternator and excessive AC ripple?

Seems reasonable to me, although .9 is somewhat high. Clip onto the alternator output stud and check at idle, at fast idle, and with headlights on and off

Reading at the ballast may be introducing coil noise as well. Do you even have the radio suppression cap? Should be from coil+ to ground. This actually is somewhat important and CAN AFFECT spark output to some degree. It proves a low impedance AC path for the spark pulse in the primary circuit. As the spark pulse forms, the primary current collapses (with the points open) and the pulse current must have a low impedance path. One end is the cap in the distributor, and the other end is the ignition battery supply and the supression cap.