Custom Auto Sound Alternative?

You list gobs of stuff we don't really want to know about your car but you forgot to mention whether you have a floor console or not.
This makes a difference. If you have a console in very nice shape I understand your desire not to cut it. In that case something like "Secret Audio" might suit you. Other folks have put head units in the glove compartment and even in a "flip down" setup under the dash. Some time ago I had a "small chassis" Sanyo (and later a JVC) cassette player in my 68 years agoand was able to shim and space it to fit an unmodified bezel. I did have to use a filler around it to cover the top and bottom gaps. Didn't look that great but worked fine.
If you instead have the regular radio faceplate, it's no big deal. I found one that somebody had already cut some to fit a cassette player at a swap meet for $5. I cut it even more to fit a Kenwood Excelon CD player. If I take out 4 screws and unplug the wires I can remove the whole setup and stick my 67's original AM radio and bezel right back in and nobody would ever know it had been changed. Works for me.