I agree with the above posters mostly. The first rule is only replace what is rusted, With a little margin. If that is more than one piece, probably better than one long piece. The donor cab piece is perfect for this. Some shape, same thickness. No custom shaping needed. I assume the windshield is removed. Can't operate with that in. The idea of removing at the seams, a bigger piece, making your own piece, will lead to a nightmare. There are two walls, inner and outer. You will want to be VERY CAREFUL about warpage. You will warp some but use the air cooling method not water. Any time there are bends or curves or dimples on a piece, it will warp less than a flat piece. Go slow. It will grind and you will filler and sand smooth just like any other repair. Don't know why you say it won't look good.
I too am very concerned about why it leaked at that point and only inside. I am hoping it was the windshield rubber leaking. If you are sure there were not rust holes above or previous owner drilled and plugged holes for lights, then it's probably the rubber.
Also sometimes when you sand down to metal just before cutting out the section(s), you will find more than you thought. It is OK to fill small pit holes with glass reinforced filler on the inside of cab. It's all cosmetic in there. No moisture except condensation. Don't mess with the pinch weld that the windshield rubber fits on or mess with the windshield opening itself.
I hope you mean rusted thru, not rusted or surface rust, You don't say the degree of damage. Try NOT to have one long piece.
Cut pieces off the old cab in various spots or cut slits with a 1/16 blade and practice, practice, practice to get the right settings and build your technique. Be sure to sand down to bare metal in all areas to be welded and sand a spot for the ground clamp. In this case you can not back up the weld with copper so if you blow thru, you will be in slight trouble. Even experienced welders blow thru on thin sheet metal. That is the only scary part of this. That is a tough place for a beginner. Your "position" will not be optimum.
You will always have a piece to practice on when doing patching. The piece you cut out. Set welder voltage to chart for that thickness metal and wire size used. Set speed on real low. Start practice weld while turning up speed to hear a sizzle/cracking/buzzing/bacon fry sound. Then your done with set up. Most times it's better to set voltage a little higher than chart and slightly vary the stick-out to adjust. I have learned a few more things about sheet metal welding if you need any help.
