I have yet to see any GM truck, or any other brand for that matter, up to around 1998 with a painted frame. I've literally been under thousands of trucks and cars over the last 22 years as a professional mechanic, no paint. Lots of black phosphorus, though. The only painted parts I've come across were control arms and in some cases, springs.
BTW, my '55, which was an untouched Southern survivor when I got it, had no paint on the frame.
