I have seen guys tack weld the replacement panel in with a mig then grind it down. Wouldnt there be a open seam doing it that way?
Lots of little tacks with good penetration and very careful grinding of only the proud part leaves plenty of metal
This is how I do all my body panel replacements since I do not own a TIG. This is a gas tank filler hole on my 41 PU, I needed to make the hole smaller to fit a flip top bung that I also welded in. Lots of tacks, moving around to spread the heat out (I don't make two tacks next to each other until you can touch them without gloves. Repeat the tack and grind process until they are all filled in. Grind and metal finish. It isn't fast, in fact the slower you go the better as it reduces warping form the heat.
If you check out YouTube look for videos from Fitzee's Fabrication; he can show you the quickest way to pick up the method of doing this.
This is how I did the quarter panel section on my 53 before I bought my new tig. Leave a gap the thickness of the metal and tack it in. Weld between the tacks a little at time.
I always use Sait brand cut-off wheels, very high quality and long lasting. I like to use the thinnest wheels like these. https://www.wehavegrit.com/bonded-a..._source=1&gad_campaignid=21258976960&gbraid=0