And how do you know it wouldn't? Most of the work they would need to do in final is on the chassis side, and even then it's not as much as you might think.
They definitely need to expand some clearances for the extended length of a truck, and some expansion in the suspension area to account for frames instead of just subframes. Semi-educated guess, total of 3 months of retooling time. They're already doing work in stamping and paint. They've painted a few bodies already.