some of the GMC's use the Chevy rad support and some don't. a GMC rad support is made differently (for USA trucks anyway. Canadian GMC's use a Chevy rad support) so the cross member is also different I think. maybe the assembly manual drawings will point that out, not sure. 58-59 frames is also different than the earlier 3 years because the front clip is longer. those have the rad support mounts further forward on the frame as well to match up. 58-59 also is longer ahead of the rad support because there is a longer filler panel between the bumper and the sheet metal. the frames are the same width. I believe the frames are the same behind the front spring hangers. the cabs are the same except for the dash. the doors interchange etc. the front fenders are different for 57 because they have a different grille surround parts. GMC fenders have a divet around the area of the mustache grille part where it wraps around the fender for the signal marker light. emblem placement on the fenders differ between years if that matters to you. other than the tailgate emblem I think the box is the same.
assembly manual
https://www.trifive.com/d1/55-59Assy.pdf
here is a chevy rad support for 55-57. if you look a little deeper you can find a 58-59 rad support.
https://www.brotherstrucks.com/1955-...tinfo/RSB5000/
here is a thread that explains a few things better because it has pics. scroll down and you will see the GMC rad support.
http://67-72chevytrucks.com/vboard/s...d.php?t=754736