if its a d1, i dont think yu can get replacement chassis for them . D2 - yes.
Gap, heat, wire speed and preparation.
Weld it up mate and add some strengthening plates to cover your seams.
Weld as normal and put strengthening plates of outside? That will knock everything out by s couple mm when I weld the spring seat and boot support posts back on?
If you're struggling with penetration on lower heat as not enough juice then bevel the edges and use the 1/3 land 2/3 bevel 1/3 gap technique, on thick stuff this is where you run a root pass fusing the lands and then a circular filler/hot pass to fill out the bevel and then cap it off to be ground flush, on 2/3mm a bevel and a gap with one hot pass would be enough I think.