If the original radio was removed and a larger hole was cut into the dash (ouch) for the installation of an aftermarket am-fm/tape DIM, is there any other way to re-install the original radio other than swapping in a new lower dash?

You can go to a stereo shop and get a new face plate and a metal backing to hold the knobs, but usually it's used for a new type am/fm stereo cass/cd that will also have a new faceplate. The only drawback in your case is you'll see the screws you'll need to hold the new faceplate that's screwed into the old one. But I guess hot glue will work.
I had to do it for a aftermarket radio.

Mark

As far as the holes for the knobs go, there is a company that makes replacement "towers" to glue into a modified dash. I think that it was covered in a Chevelle magazine in the last 2 months or so. No mention of how to address the oversize hole for the face plate area.