1973 Plymouth Duster repair and rebuild

Regarding the welding and repairing of the drilled holes, I am considering welding the 1/2 and 9/16 holes closed. Then grinding them flat. My other thought is use my hole cutter to cut little doughnut pieces of metal which will fit the hole, then weld around. And again still grind flat. Thoughts?

Keep in mind, I am a unemployed Server System Engineer, not metal fab. I do have a good mig welder with gas that runs on 220v at 40amp, and it makes me look pretty good. But I am only guessing on what power and wire feed settings I should use based on research from the internet and the supplied owners manual.

I am considering putting 2 tack welds on each arm of the hoop, then pay a professional mobile welder to complete the job. Even still, I have holes in the chassis I need to deal with. Seeking suggestions.