Mack Products get my vote to! I also bought a '37 bed from them, only I went and picked it up at the factory. They are about 100 miles from here. Saved a bunch on shipping. For about $1200 You get every nut, bolt, washer, side, stripe and some of the best looking oak flooring. For $200 or so more they will put it together for you. When you start pricing parts out, I don't think you can beat a kit!
If you have never worked with 14 gauge steel before, you might want to practice on some before trying to make a bed side. You don't just go cutting this stuff very easy. If you use a torch, it won't be flat when you get done. You really need a plasma cutter. You need the top edges rolled or something so the sides just don't flop around.
Just more to think about, Joe