General Joggler

AngeloEvs: You can form a 'step' along the edge of the new section (or the existing section) using a 'joggler'. 

Not cheap (for a hand tool!) but solves the problem you are experiencing. The head also rotates and acts as a hole punch. These punched holes are on the panel that sits over the stepped section and are your weld points (almost like 'spot welding'). Useful tool as it enables a quick means of adding mild steel right angle section to panels that need flanges!