[AcampoDave]

I have a Vintage air "sure fit kit" in my '66. I have regularly driven it in ambient temps around 110 degrees and still stay rather comfortable. Is it late model Silverado comfortable at those temps? No, but its decent. However, when outdoor temps are more moderate like in the 90's it's very comfortable. As for the heat/defrost part, it does work but I don't often need it. The a/c compressor runs on defrost because the unit only uses recirculated air.
I find the fan is a bit loud but I have tinnitus pretty bad and get irritated with noise anyhow so others may find it to be fine. (It's probably also par for the course when the fan is above the passenger's feet.)

I think the key to any of these systems is to have good insullation on the floor and firewall (with no open gaps or holes) and quality weatherstrip. Also be prepared to upgrade your cooling system. I had to get a better radiator and upgrade my fan to an 18 inch 5 blade clutch style.
Edit: one last detail...my truck is light colored. I think a dark truck would be an oven in my climate no matter what.