[unclebilly]

I’ve done lots of this stuff, even spliced the rear section of a frame from an 88 4Runner with front end damage into the back of an 80 hilux truck with a rotten frame. The splice was under the cab and I fish plated it. The 4Runner frame width was the same but the box section was taller so I had to be creative with the splice transition.

Welding good metal to thin rotten metal is a pain in the ass. You will likely find that your patch panels need to be bigger than you thought due to the metal close to the rust holes being so thin. Also, wire feeders need clean metal, no paint, no tar, or you will have bubble gum welds and a massive liability on your hands.

I’m not going to provide engineering advice on this, but I’d strongly advocate for fish plating any splices you do (if applicable). Also a lap joint is better for welding good metal to thin rusty stuff.

Set your current low enough to not make more holes but high enough to get penetration. Remember that a wire feeder is not a hot glue gun, concentrate the heat on the good new metal and pull the puddle over onto the old stuff and weave back and forth with your puddle.