Before I go into the details on the uprights I need to explain and showcase the experience I have with welded uprights and how they are built effectively. One of the most important elements to the construction of uprights is the fixture, and how it is built. Below is a picture of a Trophy Truck upright I built about 6 years ago, notice the fixture and main plate they are bolted to, they are all steel.
It is important to use Steel for the construction of your upright welding fixture, you need to combat the movement of your material during the welding process and using steel is the best way to combat this. The fixture will be extremely hot during the welding process and if you were to use aluminum then the fixture will move due to the welding heat. This is why the use of aluminum is not recommended for fixtures when welding is present.
We start with a Precision machined 1.5 inch thick chunk of steel, then machine the correct holes and tap the plate to provide the appropriate support for the fixture top plate. In the second picture above you can see the actual upright fixture bolted to the main block; this provides a very stable and accurate fixture that will not cause issues because it is not machined from aluminum.