I prefer to tap out rounds with the correct thread for these kinds of fittings, nuts tend to warp from the welding heat. A piece of 2 inch round 1" deep with the correct thread drilled and tapped into it makes a nice beefy chunk of steel to weld on. The main benefit is you don't have to chase the thread after welding since it doesn't warp.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo