He did A-ok.
10 guage is too thin to plate over after narrowing the rear frame rails for wider rims and tire.
It needs to be 1/4" minimum thickness I learned from Eddie. He used to work for TRZ Racing in Chicago they moved to Florida several years back.
5/16" steel thick is better.
You take out the Factory box section it needs to be 3-4 times thicker material to maintain and exceed Factory torsional and beaming strength.
Square and Retangular tubing mild steel is not that good or stong and its heavy too.
Need DOM 4130 or 4140 Chrome Moly tubing and that needs to Tig welded in.