Unless you are absolutely CERTAIN that your model uses thermal pads, DO NOT install that way.
Most Mac Pro models get so HOT, the only way to carry that heat away is with flat, close-fitting and enormous heatsink coated with a thin film of Thermal PASTE to fill in the gaps.
In general, pads do NOT have enough heat-carrying capacity to do that job. Pads are used only for really easy jobs like carrying away heat from an SSD-stick Drive.
Also replacing the heatsink paste is RARELY the actual solution, but it can cause enough hurly-burly around the processor to discover and fix the real problems.
What model-year MacPro?