You can also do a combination of square and round tube, that is what i did with the bed I built for my '91.  Having a background in carpentry the square stock seamed like the best to work with.  I did bend my own using the old school sand and pipe bender approach, you could also have the tube bends made at a muffler shop that does mandrel bends.







While 1/4" sounds good and manly, it is overkill and the weight makes it ridiculous medium to build with.  I built mine using .125 wall(1/8") 2x2, for the outer rails and inner rail(that runs over the truck frame), the cross sections across the bed(between the boxes) are 1x1x.095, for added impact strength, I welded strips of 1/8" plate along the outer bottom rail sides, similar to armor plate.  The plating probably wasn't needed, I use .125 on the sliders I make, and they have take quite a beating w/o any issues.