I had one built about 10 years ago. Its still on the truck. The intergrated roll bar/ headache rack saved my son's neck when he rolled it 3 years ago. I used 2x4 square tubing for the joists and 1/8" diamond plate for the bed. It was 8' originally but I cut it back to just under 6' when I bobbed the whole truck to just behing the rear spring perches. I can post a pic next week if you want.
The price of steel is very high right now, shocking in fact. That bed added over 700 lbs to the truck when it was 8' long. I was getting less than 15 MPG with a well tuned 2.6 liter motor as a result. If I had to do it again today, I would spend the extra money for aluminum. I'd check the junkyards for a wrecked rollback or the scrap metal yards for extruded aluminum scrap for the joists and use 1/4" aluminum diamond plate. I would also build it to the bobbed size to begin with. The best thing I did to my truck for off road use was cutting off the back.
83 mitsu flat bed 2.6l weber, 88 auto tranny, 2" body lift, 4.625 LSD, 235/85-16E mudders (33 x 9.50).