I feel it is very inportant to do the work on the frame with the frame set level. The factory assembly manual will give clearances and dimensions. The doors are the area that will get you in trouble fast. When I do a restoration I assemble the truck once in primer and check all fits BEFORE final paint.
I take the cab off AFTER all major work is done and clean up welds sand blast and so on. When removing major sections I also brace the cab in a x fashion.
Jeff your way might be OK but as you can tell I would not recomened it. You might need to shim the cab more than average when you get it back on the frame.
To sum up the best way FOR ME.
Do major repairs on a level frame.
Fit check
Fit check
assemble in primer
Fit check
Make corrections
Fit check
Jeff if you did do it as you planned I think you would also get in trouble with the front cab mounts I have found the holes in the repo mounts do not line up. If you did weld them shutand just drill them you would not beable to add a bushing from the bottom of the inside cab mount to the bottom of the floor. The factory used a sheet metal stamping. I make a bushing out of mild steel on a lathe. If you did not add this the floor would deform and not give a positive 'tight' I weld them closed and locate them off the frame. The holes for the e-brake support are often a problem.
Take your time and besure to allway check the fits as you go along, ask for advice if you get into any trouble or have any ? If you follow this advice major work on a AD cab can be done by ANYONE Ray