Not trying to sound crass but a quick search will turn up quite a few threads with some great write-ups and photos answering exactly that question.
Basically you want to weld up an X cross brace for each door opening and at least 1 diagonal from left to right. When you final weld your floors in do it with the car on the frame. Measure twice (or 2000 times), weld once, etc
Basically you want to weld up an X cross brace for each door opening and at least 1 diagonal from left to right. When you final weld your floors in do it with the car on the frame. Measure twice (or 2000 times), weld once, etc