I did something similar for the rear mounted gas tank on my 66 project so I could drop the tank without needing a wrench up in there. For that I used square nuts, because there was more meat on each corner and less chance to warp it. I also found that putting a bolt in the nut while welding helped from warpage too.
I see that summit carries weld nuts, which I might use on this project.
https://www.summitracing.com/parts/a...4aAgXgEALw_wcB