I ordered new MOSFETs and did load testing. Using a PWM generator connected to the gate of the FET and a high power supply from drain to source while the chip was bolted to the large heatsink with thermal interface material, I was able to dissipate power in the transistor to whatever degree I wanted. The originate design was to share the power between two FETs, so the chip only needs to handle 50 watts nominal power dissipation. I will be testing the first chip to see its breaking point.
After completing the test, I find that the FETs I have purchased are quite spectacular. I was dissipating 220 watts for 1 hour under forced air through the cooling tunnel and the chip show no deviation in performance characteristics. The surface temperature is below 70C, so the cooling method is working very well. I then cranked the chip up until it was dissipating 600 watts of heat. The chip continued to operate for about 5 minutes before it catastrophically failed. The chip became a direct short and this was on the highest voltage rail I had (to lower current in my wires). Unfortunately that means my reservoir capacitor was storing a lot of energy at the time and it had no issue destroying the chip.
Note to self... Don't put 600 watts through the FETs