1) Avoid making anything structural out of welded aluminum. Welding puts it back into its annealed state, which is VASTLY weaker. If you can, at least assemble everything with large bolts.
2) 6061 Aluminum is about as strong is typical Hot Rolled mild steel (its actually a big stronger in terms of bending). 6063, however, is only about half as strong, and is mainly used when dimensional accuracy and appearance is of greater concern than strength (like if you were making drawer guides, and you wanted it smooth and strait). 6063 is produced with a machined finish that is smooth, with sharp corners.
3) While aluminum can easily be as strong as steel, its weak point is its fatigue life. If you made your item out of steel, it can flex a bit when loaded, and for the most part be fine after you remove that load (to a degree). However, due to Aluminum's brittleness, everytime it flexes, some of its bonds are broken without reforming (as opposed to the steel's greater ability to "stretch" its bonds). Basically, everytime its flexed, it gets weaker. For example, to break a paperclip by bending it back and forth my take 20 bends. For an aluminum one with comparible strength, however, it might break in the first one or two bends.
In a nutshell, to build something out of aluminum means more thought must be put into keeping the assembly as rigid as possible, to avoid this fatigue. Be generous with triangles and you would be fine.
You could use the same dimensions of aluminum (maybe step up to .25" thick instead of .1875", in more critical places), but use bolts wherever possible, instead of welding.
Like here, for example. Instead of welding the top piece to the bottom. Pass 3 or 4 bolts (at least 1/2", grade 8, since the stress would be all tensile) vertically through both tubes:
Also, on the supports for the main column, would would more likely want to have them go all the way from the ends of the legs to the top of the column (or at least as far as possible, but at the same time try to stay close to a 45* angle), instead of just part way down each axis like here:
However, even those would probably be ok how they are. But again, instead of welding them, attach them to the legs and colums with a flat piece of probably .25" thick 6061 Aluminum bar that extends from the sides of the angled supports out, to straddle the side of the main column so bolts could be passed through them.
The largest problem by far, however, is the lack of support on the rear of the column. You would need to extend the base rail out past the rear of the column. A foot extra would probably do. Then, just add another support like on the sides.
Here, Ill make a few images real quick.
*edit*
Here is the view from the front. It is not really to scale, especially the bolts heh. Note that the rear support is mounted higher on the column (as close to the top as reasonably possible) than the side supports, because it will have the greatest impact on preventing forward flexing when the load is applied. Also note how the spindle part passes THROUGH the column, rather than over the top and welded. You could pass it through the column, then weld it, and it would still have the same strength of the surrounding metal.
Here is a view from the top. The same applies, but you can also see how the rear support would be roughly the same as the side ones, just longer.
