I built my own deck, and when I did, I had a GMC short-box, so when I went back to dodge, I had to move the rear legs forward a couple inches. I have a 8' flat part of my deck, with almost 2' of beavertail.
As far as the tie downs go, yes, the factory plastic ones suck. I got 4 large eye bolts, welded the eye closed, and drilled through the box on the bottom, near the sheet metal strengths ribs. I used large aluminum plates (3"x5", by 1/8" thick) washers and lock nuts. I wanted to spread out that load real well. I can take some pics, if you want to see what I did. It works pretty well, and there is a LOT of force on them when loading. I also installed airbags, and run e-rated tires.
When I had my GMC, the front end was real splashy with 2 sleds back there, that darn dura-minimum was so light. But, that Cummins up front now, keeps that front end on the road much better.