For direct flame impingement? 309 stainless steel filler with a tig would be my first choice just because you can move the slowest with it on a tig. If you haven't used 309 before, practice with it first it's a weird looking puddle.
Usually flame burn out failure on steel means more leaks are just around the corner. If so you can cold bend new sch 40 around a pipe about 2" smaller diameter than your desired size without sand or ice packing and wasting icy & fuel. I hate to see what a couple of 21' sticks of 3/4" costs up there compared to here.