If you are getting grainy images on the D70, there must be something else going on to cause the problem. It could be applying excessive sharpening, contrast or using too high an ISO for the camera.
There is some benefit of resizing D7000 images that reduces noise, and there are other positives with the camera. But there is about 4 stops difference in exposures of the D70 vs. D7000. You can certainly get 3-4 second exposures on your D7000, but it requires some adjustment to camera settings and very bright light.
Both of these cameras can be used for IR under the right circumstances. I'd encourage you to do more testing. I find the sweet spot for IR images is around f/8 - wide open and the focus shift is hard to manage while stopped down tends to produce hot spots. You want to use base ISO - ISO 100 to ISO 200. And you need to watch your post processing technique to avoid creating noise.