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As a cantilever?
I will assume that the load is uniformly distributed and limit the look to the capacity of the HSS. You will, however, need to consider the connection and whatever is supporting the cantilever.
That can hold approximately 1400 pounds uniformly distributed over the 30" and will deflect approximately 0.2" (assuming an infinitely rigid rotational support and connection)
As long as the weight isn't biased to the edge of the cantilever, it's fine. And also as long as no other loads are introduced, and as long as the welds are full joint penetration welds. There are factors of safety that make up for some deficiencies, but you don't want to rely on them in that way.
The cantilever can definitely handle the load. The rest of the frame appears robust enough to handle it, too, but I can't say for sure without doing a complete analysis of the entire frame....and also ensuring that it meets the overall stability criteria with the factor of safety of 1.5
It also depends on how the base lays out compared to the load for the stability.
I don't know what you mean with the first part.
The base should be wider than the top in all directions to ensure stability
Not just bigger, but the base must be wider than the arm in all directions. It can be bigger and not extend further in all directions.
Then that will be fine.