I know all of this information exists across a number of threads but I didn't find one that laid it out clearly enough and I made a bunch of mistakes. So I thought I'd share exactly what I did to get rid of the stock cooler that comes with this system using the existing backplate and without removing the motherboard.
So here's what I did:
1. Bought a Noctua NH-D9L cooler
2. Bought four 3mm x 10mm screws and lock washers. (Do not try to use 3mm computer screws as they are too short to reach the backplate posts below the spacers. Computer stand off screws are 3mm and 6mm long. They are also not long enough to reach the posts on the backplate.
3. Remove the old cooler and thermal paste
4. Place the black spacers provided by Noctua over the existing posts.
5. Screw the mounting plates (threaded side up) on top of the spacers using your 3mmx10mm screws with lock washer. Be sure to orient the plates so that the cooler fan will blow front to back.
6. Apply included thermal past to chip.
7. Install the cooler and fan according to included instructions.
8. Marvel at the decrease in noise and CPU temps.
My results:
Before:
75 degrees CPU temp at idle
100 degrees CPU temp at full load (at least until it starts throlttling which is almost immediately)
Loud af nearly all the time
After:
45 degrees CPU temp at idle
71 degrees at full load with no thermal throttling (still throttles when the 3070 is working hard due to an underpowered PSU IMO)
Almost silent at idle and medium load and much quiter under full load