From an engineering standpoint, if the cross member is properly welded in, you should have no issues. But you need to be careful of undercutting or introducing too much heat and changing the metallurgic properties of the frame around the weld.
Back when I was researching this myself, I found a thread (which of course I can't find now
) about bolts vs. welding, and as I recall the consensus was that our frame rails are too flexible (unless you box them) to weld the cross members in, that it will just lead to cracking of either the welds or the frame near the welds.
I would be inclined to believe that, both based on the amount of flex I've seen in the trucks, and because when GM started welding frames together was when they started using hydroformed box rails and heat treating things.
Food for thought.
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