This thread still has the info for those that want to perform the retrofit.
GMT-800/900 cabin air filter retro fit kit using GM OEM Parts
For those that don't want to read, here are the relevant part numbers.
GM Part Numbers (GMT 900 series straight bracket)
22759208 - Retro Fit Kit (bracket & screw ONLY, no filter)
22759203 - Cabin Air Filter
23101674 - Cabin "Carbon" Air Filter
Only 1 type filter is needed
Dorman Retrofit Kit part number (GMT 900 series ONLY)
259-200 - standard filter w/ HVAC bracket & screw
259-201 - carbon filter w/ HVAC bracket & screw
GM Part Numbers early NBS's 1500/2500/3500 GMT 800 "L" bracket type (Older style)
52494070 - "L" Bracket Cabin Air Filter Cover (not sure if screw is included)
19257782 - Cabin "standard" air filter (QTY 2 half filters)
12489479 - Cabin "Carbon" air filter (QTY 2 half filters)
Only 1 type filter is needed
Notes/Maintenance :
Going forward replace cabin filter/s once a year.