Another overheating situation 63 Slant six

1) Does the lower hose have a spiral anti-collapse spring in it from end to end?
2) If there appears to be a good bit of flow to You at idle, but winging the throttle causes coolant to shoot out of the neck, it would indicate a restriction in the rad/lower hose. Usually the coolant will try to come out of the neck on a sudden closing of the throttle after holding an elevated RPM due to inertia coming out of the head/upper hose & the abrupt lack of pump depression at the lower hose.
3) RRR is correct in that retarding the timing will waste/reject more heat to the coolant & exh. But 8deg initial w/the stk amount of mech advance shouldn't ever cause this much of an issue.
4) Where You live & the current temps, it should take a little bit of driving just to reach full thermostat cycling....fan operation be damned.