I think it depends on how solid your cab was before you started. I've always thought if any major reconstructive surgery was required (floor pans, floor brackets, structural pieces, etc.- the body should be left on the frame, assembled. Then taken off after the welding work is done. This is especially true for convertibles, off topic, I know. The reason should be apparent, if the body isn't structurally solid, you have no reference point of where things SHOULD be while on a dolly, and it will also flex. So, at this point you probably SHOULD put the cab back on the frame, and shim it close to where you think it needs to be, before doing any more cutting and welding.
