Aloha,
I've not worked on a TD body tub, however I've worked on my TF body tub. I gather from your question you are repairing the metal skin and not the wooden frame. The only place that I know that is welded is where the scuttle panel meets the front quarter panels. Screws and nails are used to hold the skin to the frame. If you are going to do this with the tub on The car, you will probably need to remove the fenders and running boards and some or all of the interior panels get to the fasteners. Do you think that any of the wooden pieces will need to be repaired or replaced?
If you are removing the tub from the car, I would suggest you build a jig to mount the tub on to keep things in place as you remove pieces. Something like a large pallet that the main side rails can be attached to. Take lots of measurements from side to side, diagonal and left to right to assist you in reassembly. Rebuilding the tub is a big job.
If you are repairing rust damage on part of a panel, most of the panels are virtual flat sheet metal. Cutting out the bad, flanging the edge and welding a repair piece in should work. Care needs to be taken to not to set fire to the wooden frame when welding, in addition to all the other welding concerns.
Don't forget to use a metal preservative on the inside of the tub when you have made the repair. Something like Waxoyl will work well.
Good luck. If you have the time, please let me know what part you are working.
Safety Fast,
Dave