Moped Army

So I want to replace the gila exhaust studs with socket headed bolts because when I try to put nuts on the studs the nuts can't rotate due to hitting the pipe. But these studs seem to be attached via some black magic. Looks like an anchor of some kind they are screwed into? I tried welding a nut on the end of a stud and a bit of belabored turning the weld broke and I saw that all I did was bend the stud a little. Whats the deal?