Well - update. The OEM part number for these is obviously discontinued. I asked the parts counter at my job (Chevy dealer) and they gave me a Delco alternate number of D7085. You can apparently still order this from Autozone.
While there I also found a Help/Dorman kit 13808 that has a few different retainers in there.
One of them let the switch screw in just fine. I'll find out tonight or Saturday whether or not it fits the pedal bracket, but if so - I'm good and have brake lights now.