Thanks CP66!
Good information!
I am pretty sure that the dash itself does not require the cutting of anything to fit the average, decent 2 post GM style radio (intended to fit the rectangular opening in the actual dash structure), but the woodgrain/chromed plastic trim piece was often cut for installation of aftermarket radios, since those units often filled the entire rectangular dash opening with the display area and a cassette opening or something similar. Sometimes the owners would perform other significant modifications (Butchering?) of the dash structure to fit a larger aftermarket stereo, but I have repaired the dash structure to the point that it is now pretty close to appearing like an unmolested '69 dash. Since mine is a resto-mod and my theme is to keep as much of the "flavor" of the original '69 GP as possible, but add some enhancements I should be set up pretty well to accomplish my plan. OPH answered my primary question about the overall fit of similar radios, and it would appear that any aftermarket radio which will fit the basic dash opening for the GTO will also fit the GP, allowing for modifications to the trim facing the passengers. By the way, there are several guys (like Kevin Wagner of Wags Klassic Electronics -
http://www.facebook.com/groups/613613486896043 ) who will repair (if necessary) and add bluetooth functionality to your factory radio. It's not cheap, but it can really add to your driving experience if you are driving longer distances.