Here is a page of stats of various aluminum alloys
http://www.onlinemetals.com/aluminumguide.cfm
Though I get about 85% stronger for 7075 than 6061 (for yield strength, which is what you care about) using these numbers. The thing is, you probably don't care about strength. I'm guessing you are going to have enough "meat" there that the material itself is not going to shear or deform. Your probably most concerned with 2 things, stiffness, and dampening. Stiffness is roughly the same for any aluminum alloy you get (take a look at the "modulus of elasticity" or "young's modulus" for materials to compare, bigger is stiffer). You can get stiffer by going to steel (it's 3 times as stiff as aluminum) or cast iron (about 1.5 times as stiff as aluminum).
http://www.engineeringtoolbox.com/young-modulus-d_773.html
Ok, now lets talk about dampening. I'm not aware of a metric for dampening, but cast iron is generally considered very good (one of the reasons machine tools are made out of it), and steel is considered poor. Try an experiment. Take a piece of steel and a piece of cast iron roughly the same size and shape and tap them against something hard. The steel will "ring". The cast iron will make a dull thud. Try it with wrought aluminum, and it will ring, but probably not as much as the steel. I guess what I'm saying is, aluminum is probably a "middle of the road" choice, and the alloy is irrelevant (though, cast aluminum _would_ be better, as it has better damping than wrought aluminum).
Hardness may be a concern, but most of your wear is likely going to be on the bottom, and you could re-surface that. If you are worried about cosmetic damage on the visible portion, you could annodize it, and that will make the surface _real_ hard. You could also mount it to a steel or cast iron "foot" for mounting, I honestly don't think you are going to run into issues for years if at all.
As for pressing the bearings in, I think you will want 1-2 thousands interference. Make sure the bearing pocket is cool when you take your final measurements, aluminum expands a lot when heated. You will also want to chamfer the edges to ease entry. Make sure you press on the outer races when you press it in, not the inner race. If you are not going with shielded or sealed bearings, make sure you have enough room to work in an external seal.
Have you chosen bearings? Have you figured out how you are going to pre-load the bearings?
