well the standard answer for this is "depends"
given proper prep, joint fitment and design, as well as good penetration, the weld will have a strength of 60, 70, or even 80kpsi, depending on the filler used, so a 400# constant load should not be a problem.
However, remember if you are using pipe, schd 40 stands for 40kpsi, and schd80 stands for 80kpsi.
Can you provide a drawing for what you are doing? The point of failure is most likely to be in the shape or design of the side piece. If merely a linear "pull" on the cable to the side piece to the post, the main area of stress will be the attachment point on the side piece.
remember, fabrication, as well as all actions in life depends on the following:
cleanliness, lubrication, and penetration