How did you do it?

I am in the process of doing this today on a 69 Dart. What year and model are you working on? For mine, the patch piece has to be clocked to match the original. My gas tank is secured with bands that hook to tabs on the bottom of tire well so depth is important too. Also with the correct depth issue, my gas tank overlaps (or rather underlaps) the tire well.

I first measured the depth of the existing pan, and I marked the orientation of the tabs to the frame rails. I am cutting out the old pan today and matching that cut to the patch. I haven't decided if I'm going to butt weld it or overlap it. Either way, for me, it needs to be the same specs as factory.