New to bodywork, suggestions, and advice

[MP&C]

Any weld on sheet metal is going to shrink, so first and foremost, look for access from the inside/rear side to be able to use a hammer and dolly to add some stretch back in the weld and HAZ. On parts like the rocker, don't worry about cutting a part of it, use the entire part. Panels like the rocker that don't have access for planishing, you are better off using the entire part and using plug welds to attach, which won't have the shrinking issues that a weld seam will. So worry less about using the least amount of new metal as possible, and more concern with planishing access, or methods that provide less shrink.

If you do need to add a seam, make it near a higher crown area or a crease, as these areas will help to minimize the warping effect. If your patch panel has a stepped edge for lapping under, cut it off and do a butt weld. Lapped seams are prone to ghost lines in your finished paint.