one very good way to do this with out filler or welding is to glue a patch in. i know it sounds crazy but the glue holds better than a spot weld and is becoming a warranty procedure repair. cut your patch a lil big grind the paint on the under and upper sides, trigger some glue out, stick it on, clamp it, and take a plastic filler spreader and spread the glue like its filler, sand it down and you have a solid repair.
sorry for the long explaination: automotive body major with 4 years experience
that engine bay looks awesome how long did it take you to do it unfortunatly for me i am of limited body work skill and and patience but that looks sweet