That will depend on the aluminum alloy. I don't have any engineering data on aluminum. A steel 35ksi W8x10 at 8'-0" span simply supported at each end will hold a 7500 lb point load without exceeding the allowable stress of 24ksi. A W8x10x8'-0" would be relatively inexpensive, and will only weigh about 80 lbs. A W6x12 would hold a similar load and be a little shallower, but it will also be a little heavier.