Speaking from experience: you may want to add a small triangle that goes from the side bars down to the flat iron on your bed.
When I built mine, it felt like it wanted to bend when I was really using it. I used some hellferstout materials, but it just wasn't as strong as I wanted. I added a small gusset on each side, about 1" is all. It was amazing the difference it made. I've actually winched an elk into the back of the truck by going over the top of it and pulling sideways.
You may never have a problem with it, but it is something you might consider.