T420 support GPU card on slot 3 and 5. These slots support up to 75W. Please refer below link for details and guidelines for GPU installation on T420. [URL] 
Is anybody able to tell me the max wattage for the PCIe lanes compatible with GPUs? Also, is there anything in the BIOS that could prevent full power delivery to these lanes? I have a 75w GPU that does not rely on any external power. I have 2x 495w redundant PSUs in the system.
Poweredge t420 PCIe wattage for GPU