I'd be grateful for thoughts on the following:
I'm about to repair the chassis box section of a recent toyota - which I believe is made from a high strength (and low alloy?) steel.

I hear seam welding this material produces non-ductile welds that will shear under stress - and that therefore spot or plug welds are preferable.

1. One part of this repair will join two box sections together, my thought is to MIG stitch weld the join (to keep the heat down) AND to reinforce the area with secondary plates inside the box section plug welded to the original box section. Will this be a good enough repair?

2. I also want to introduce new material to the box section to replace some of the crushed original. Obtaining the original material is unlikely so can I introduce ordinary mild steel in places and join it with the high strength steel? Are the any bad structural impications of doing this?

Thanks in advance!