if you look at Chuck's site, you will see that the the support for the front of the cab is provided by thhe toeboard,, floorboard,, inner cowl, outercowl,inner to outer cowll and the hinge post, all together. You may be able to pinpoint where yours has collapsed and where it needs to be repaired. I would think that you would want the fenders detached because they bolt on to the toeboard. Chuck used 2x4s attached to the firewall and resting on the frame for support. I would think that you would want the doors off too. It is important to take care that the cab doesn't distort during the work. You should check to see if your doors are meeting with the body, along bothe back and front vertical edges. It is possible to remove the front fenders,, grill and rad saddle all as one if you have some help. It was actually designed to do that.
There was a post a while ago showing a partial repair to these areas. I will keep looking until I find it.