I did the over heat to reflow the solder before removing the heat sinks and adding new thermal paste if you do it after adding new paste the heat can mess-up the paste (I powered the 360 for 25 min. with the fan sitting on top of the two heat sinks blowing the air up IIRC then let it cool for an hour or two). Also I never heard to "torque down" the screws that would either cause the board to bow or it would squeeze all the thermal paste off the chips you don’t want it to tight. Are you screwing it down in the right sequence? You want to get all the screws even rotating every turn or so till it gets to the end then go with a quarter or less per turn like this.
1 3
.X
4 2
(Ignorer the . needed it to space the X)