1.5x1.5 stock, at those lengths, are not particularly strong, when one starts putting MANY hundreds of pounds on it. Id suggest a simple diagnol brace from the main upright to the front or middle of each arm. The middle would probably be ok, if the brace is at a 45'ish angle. It wont interfer with his loading them up and it may keep him alive and undamaged.
Did I mention I work in factories all over So. Cal, and have seen stock racks, home made ones, collapse like the World Trade Center?
But hey...I was only making observations. Take em or leave em. Shrug
And yes..you can overload anything. Sometimes however..its harder to do when its properly braced or made of the proper materials. Shrug again.
Gunner, Owner Coyote Engineering