I am surprised that you were able to upgrade the processor. The problem may be that the cooling system was not designed for the upgraded processor and is not able to cool it properly. The heat sink compound on the GPU may not be contacting the heat sink surface, the silicone heat strip is thicker and would contact both the GPU and the heat sink. At a minimum, put the silicone strip back in place.
I would recommend going into diagnostics to test the CPU, heat sink and fan. Press F12 at the Dell splash screen and pick the menu option for diagnostics. The test you want is the CPU stress test, if the temperature get above 80 degrees Celsius ( water boils at 100 Celsius), then the heat sink is not sufficient for the upgraded CPU. You can try to get another heat sink asby and fan to see if it improves cooling performance.