Thanks for your comments, here's what I ended up doing:
I first tried removing the gigabyte logo (NB) and replacing the thermal tape (that comes stock on the board's NB/SB heatsinks) with good thermal paste. Only the tape on the NB had apparently gotten hot enough to melt. Afterwards, the NB heatsink was still too hot to touch (maybe even seemed hotter than before) with the PC in idle.
I bought a HR-05 IFX and also an HR-05 IFX-SLI. For the NB mount, I tried straightening out the longest HR-05 bracket (comes bent), but ended up accidentally breaking off the end. I broke off the other end, too, and used those two end-pieces to extend one of the other short brackets by fastening the three pieces together with some MB mounting nuts and screws. This kept the bracket's dimple in the center. The resulting mount seems fairly stable, although the heatsink can be made to twist a little about the mounting bracket's dimple point. The HR-05 is dwarfed by my ninja cooler, but seems much cooler than the stock heatsink.
I couldnt find a good location to mount the HR05 IFX-SLI to the SB chip, the heatpipes were too close and/or interfering with the capacitors and USB pins on the motherboard. One end of the retaining bracked also interferes with the IDE cable slot, but the bracked could be trimmed to make room. The stock heatsink seems to get too hot for its own good, but I'll leave it alone for now.
Temps as I'm writing this (fanless--all fans off) are: