An exclent source for materials data is
www.matweb.com. They offer data on a wide range of materials, and information on different alloys, heat treatments and even specific dimensions of samples.
The relevant pages for the two alloys, both with a T6 treatment.
6063-T6
6061-T6
Of course, its all refrence data, and they keep a disclaimer against using that data for design. Ideally, you would do your own testing with samples of the material as it comes from the supplier. For FIRST purposes, its fine to use handbook data like this.
Testing methods vary from one source to another. The important thing is to pick a reputable source and stick to it. I think there are a couple 'machinist handbooks' out there that would probably have this kinda of data.