Interesting update:

I spent the afternoon with the car. Pulled the thermostat out first thing. There was so much water flow that I couldn't rev the engine much without throwing water out with the rad cap off. Excellent water flow.

It was very slow to even get to 150*. But I ran it a good hour or so and blocked some of the radiator air enough to get the temp to 180* or 190* and then pulled the cardboard so that it went back down to 150* or so.

No temp spikes or thumping or blow outs.

Hmmmmm.

So I'm guessing that the pass side comes up to temp first and maybe gets too hot before the thermostat opens. Maybe so hot that it creates a steam pocket. I always drill a hole in the thermostat, maybe I'll add another hole or two.

Now I wonder if that pass side is lean on a cylinder or two. The tubes are roughly the same temp when I check.

More testing in a day or two. I'll drill more holes in a thermostat and see if that allows enough water flow to keep the pass side from overheating while it warms up. And also monitor the tubes on that side for problems.