I am about 2 yrs behind on this science.......up to then I know frames can be welded you have to use a heat stick to moniter the temp ( most want 600deg b4 the weld starts and slow on the cool dwn. Asub frame will be made out of reg. steel and will add streath no matter how you do it as long as you stay away from ac welders use dc and wire feed for most of it as it will cause lot less stress.
semi trucks prefer not to be welded but it is done every day ,,they are stretched shortend and fixed all the time.
For myself I would not be scared to go do it.