Fairly sure you will find its the intake gasket thats causing this, but one other thing to check is that the inlet rocker studs have had sealant applied where they screw into the heads, IIRC the AFR heads are not blind in this area and can pull
oil down around the threads if sealant is not applied. The point you make where the pinging only seems to occur after a period of high manifold vacuum tends to point in an
oil leak around the intake port of these cyls.