My chamber support bracket has three flanges. I added some cracked-weld pics: S-Cam Repair & Cracked welds
5/8"-18 is what's on my axle tubes - the threads were badly butchered. I still haven't found a 5/8"-18 thread chaser - 9/16" seems to be the biggest.
A thread-chaser is not, and never will be, a tap
A thread-chaser re-forms the existing threads
Dulling a tap can make it a thread-chaser
Use a on a wire-wheel on a bench grinder to dull the tap.
Be really, really aggressive in dulling the tap - you want the "dull tap" to re-form the existing threads, not cut new ones.
With fine threads, false starting the dull-tap/chaser is easy so take your time
If the hole is beyond saving, there's always helicoil repair - which is a lot easier said than done for a 5/8" bolt, on really hard/tough steel. Not a job for the biggest 1/2" drill.