Over the years, I have welded several of these. I tried simply grinding and butt welding the arm back together the first time, but it developed cracks and broke off again 2 inches from the weld. The next time, I made a backing plate to fit the inside of the arm. This time, it held and is still holding. I would suggest using a 150 degree preheat, and wrappiing the arm with some sort of insulation to slow the cooling process. Also, keep the innerpass temp to about 200 degrees. Take your time and dont let the weld zone get too hot. I use 3/32" E7018 and make sure not to weld with more than 3/4ths of the rod to avoid any more heat input than neccesary. One other factor to bear in mind is sometimes, these arms have lots of hydralic fluid on them from broken hoses, blown seals, etc. Make sure you remove all traces of oil if you find any.
Good luck.
Mike