I'd just cut it flush then grind down, with what you need to weld on and go from there.What about sanding it down flush with the cross member? I'm just going to cut them off completely and go with adjustable bolster mounts, so i can fine tune it to my liking. Is a flap disc the best? I do have an air powered sanding disc like what you would use for body work
I doubt the original rail was hardened considering aluminum. At the same time I am not a pro aluminum welder so best advice is consult someone local who is.