Grinding aluminum is difficult and messy. It can be done using specialized wheels. Much better to cut the piece.
I have access to one of these
Power Tools - Band Saws - M18™ Cordless LITHIUM-ION Band Saw Kit, but it has a throat capacity of just over 3 inches. Larger band saw with proper blade or even a sabre saw (scroll saw) would work, although you would have to be extremely careful with it. After cutting, sand the thing with coarse grit, if the cut is not perfect.
There are much better ways to cut your piece, but the sabre saw is quick and dirty. Most people who have tools have one. Just make sure you have the right blade for the job and a steady hand, some patience, and a good eye.
Another possibility is using an angle grinder with a specialty disc, but again, it's going to be messy. Much better to cut than to grind anything more than 1/2 an inch of material along its length.