Overheating issue with the old slant 6

These old engines have a very simple cooling system. The thermostat goes spring side toward the engine, not the radiator.

I don't recall if I saw the water pump mentioned. Sometimes the impeller can come apart without showing outward signs other than the engine not cooling well.

Either way, with a new radiator, new thermostat and coolant circulating there is almost no way it can overheat unless something worse is going on like a blown head gasket. It should run about 180 degrees all day long.

I absolutely would get that radiator out of there and exchanged. Tell them it split the trans cooler and you need another one, no discussion. Be glad it didn't pump coolant into the trans because that is BAD news.

Get the new rad in, new thermostat, fill it, run it and let it burp a couple times as you fill the coolant and see what's going on. If it's still going up/boiling you have something worse going on.