If you plan to machine a wheel, why 6151? 7050 stable plate might be a better choice. Lots of large airplane parts that start as 200-300 lbs and end as 20 to 30 use that one... removing lots of material sometimes make parts warp. The Yard in Wichita is also a good spot for thick al plate. I think most of it is around $1.50 a pound.
btw, Bob at ALRECO is a landspeed racer, ran a roadster I think.... still goes every year...