I have a 3" body lift on my 1974 K10, and a 6" suspension lift, to clear 38" TSL radials. The front flexes very well, and I still had to trim my front fenders a bit. The truck sort of evolved over the years, and originally wasn't purpose built for off road use only, although that is what it mainly sees now. I have no regrets on the body lift, but I don't think I would go that high again, maybe 2" at most, and mainly only on a street truck.
The biggest con (to me) of the body lift is that it does raise the center of gravity quite high. I have never rolled the truck (yet), but it has fell over on each side. I thought I saw a sticker that said 'chicks dig body damage', but I haven't scored one yet based on that!
As was said, the frames do flex a lot, but the factory body mounts had rubber discs in them, so I think they absorbed a lot of flex instead of transmitting them to the sheet metal. I do have some cracks and tears in odd places on my sheet metal, but I think that is due to not frequently checking tightness of body components (there all dented in, why bother now), other than cab and bed mounts.
Good Luck!