250 I6 Overheating

['63MurderTruck]

I plan on swapping out my son's 250 for a sbc asap.

In the meantime, I have to keep this I6 on the road for a while.

It's a 250 I6:
It doesn't overheat at idle, it only overheats while driving. This leads me to believe it is a cooling issue and not an air flow issue.

Even though air flow isn't the issue, I'm switching it over to a 3000cfm electric fan.

I've already changed out the thermostat. The old one was a 160, the new one is a 180. This didn't help at all.

While driving the engine heats up to 250 degrees. Obviously, I shut it down as soon as it gets this hot.

There is no leaking of coolant at all. No hoses are collapsing. I've double checked the gauge, it works perfectly.

It has a brand new 3 core radiator.

This is the first time that we've had this motor running, I have no idea of its previous history.

Worse case scenario is a blown head gasket. A buddy told me to take off the radiator cap rev the engine to 2000RPM and watch inside the radiator as the engine heats up and look for 'bubbles'. He said if I see bubbles it has a blown head gasket. No bubbles at all. Anyone else have any methods for checking for a blown head gasket?

I'm aware of a few issues that I plan to work on that may be causing the coolant flow issue:
Timing(if it's a timing issue should I advance or retard the timing?)
Water Pump(any suggestions on how to check this?)
I'm going to flush and burp the system

Are there any other issues that I'm not aware of that may be causing the coolant flow problem?

Thank you