I would assume that you cannot restore a RCA DVR40 image to a HDVR2. Likewise, I would assume that you cannot reatore the DVR80 image to the HDVR2 if they have the original drives in them. Correct?
All series 2 DirecTivos, except the HR10-250 and the R10, all use the same software beyond version 3.1.1c. Once upon a time there was a difference between the RID and non-RID DTivo OSes but eventually they both started using the same software. As long as you are using an OS version later than 3.1.1c they are completely interchangeable. If you're using 6.2 then there's no problem whatsoever.
The only caveat is that the size of the drive the image is being restored to must be the same size or larger than the drive the image was taken from. In your case, you could easily restore a DVR40 image to an HDVR2 since they both came with 40GB drives. You cannot restore a DVR80 image to an HDVR2 because the original image was taken from an 80GB drive and you'd be trying to restore it to a 40GB drive so the image won't fit. All of this is assuming you have the stock drives installed, of course.