Working on my granddaughters 94 SL 500 and wondering if anyone has encountered my problem.

The D... plastic radiator broke and took out the head gaskets. Replaced the gaskets and the radiator with an ALL ALUMINUM radiator. First thing I encountered was that the left side of the engine did not fill with coolant. fortunately I figured that out before it did any damage but the only way I could get it to fill was to disconnect the heater hose at the rear and pressurize the coolant tank to force the coolant over.

That seemed to fix the issue but when I drove it it was fine on the surface streets (40-45 mph) running about 80-90 degrees. As soon as I got on the freeway (65 +-) the temp instantly started rising hitting well over 100 in less than a minute. got off the freeway, took surface streets home and the temp settled right back down.

My first thought was that possibly the thermostat was not making the changeover from internal circulation to circulating through the radiator so I attempted to pull the t stat housing and two of the three bolts seized in the aluminum. My immediate response was severe violence so I just parked it in the side yard until I get over it. Sometime this week I will pull the water pump but in the mean time has anyone run into anything like this?

To anticipate the obvious first question I did check the new gaskets hole for hole against the old ones before installation. And the second question when I got home the coolant tank was still full but you could hear it gurgling so there was obviously a hot spot somewhere.

I really don't want to go back into the engine but I need to weigh that against my feelings for my granddaughter. Any ideas would be greatly appreciated