...L-78 used PVA, but millions & millions of GM cars used MVA. The changeover year was 1967-68.
My GTO was driven off the showroom floor, idling at 26*. 6* initial + 20* MVA. By 1968, the idle timing was 6*; the vac advance was now ported.
Same engine, same cam, same comp ratio. Going to PVA...