It is possible to remove the cab with the steering mast on the truck. As you lift the cab, you will need to gradually roll the chassis forward at the same time. I would not try to do that unless you have a way to lift the cab straight up without it wobbling all over the place.
removed engine from frame,built a temp stand to hold the engine and tranns.Gonna clean up the frame and paint before anything goes back on.Putting the 59 cab on and am seriously looking at using all the 59 front body ,fenders hood etc as the 55 is quite wonky.Will put the 55 box on and it will still be a short bed step side.
Have 55 GMC 9300 short box step side .Needs restoring but starts and drives ,no brakes, also have 59 GMC 9380 with very good cab for parts.have been doing MGA's all mjy life so this is new to me