My cab is in the same shape. The first thing I did (it's still on the frame, but am planning on removing it to do the restoration) was to bolt a piece of angle iron between the lower door hinges at the right distance per the weld checking dimensions in the FAM. Using that to jack the cab to the right position will allow you to keep the alignment correct as you weld in new front cab supports and floor. Cross-bracing the door openings would be a good idea as well.
Here's a pic of that brace I put in.
cab brace
