Round back to square back alt

ChargerKG said
So output fluctuates quite a bit. Sometimes up to 18 volts reading at battery when cold. As the VR warms up it settles down to 13.5 to 14 volts but still have wild swings of output, flickering dash lights. Driving around after hot, stock amperes gauge might show discharging for 30 seconds then back to positive

Unless either part was used successfully with the old parts I would warranty return the VR, and if that did not work then the alt.

18v is a full fielded alternator suggests a VR issue and flickering (as in rapid flickering vs pulsing) could be a diode issue in the alt.