Bradzoo is right, I did this a few years ago. So far the deck has been out west 4-5 times and to the Yoop at least 6. I have a 1 ton SRW long box with airbags (I also put a rear sway bar on this year). If I have just the sleds no air in the bags, with a fully loaded trailer I add about 40-50 psi to the bags. I know guys do it, but I would NEVER recommend putting a sled deck on a 1/2 ton. Sure you can bump up the rear suspension but you are still overloading other components on the truck, not to mention you are legally overweight and taking all kinds of risks if something were to happen (try making an insurance claim if you are exceeding the legal limits of the vehicle you are in - or will your insurance defend you if god forbid you lose control and hit someone? Too big a risk for me anyway). Just the 2 sleds is at least 1,100 lbs if you have the lightest sleds on the market. That is close to total box payload alone before you add passengers, gear, the deck, etc...
Anyway, back to my build. I started with a 92" wide Chilton sitting in my buddies back yard with a rusted off axle. Cut everything off of it except the frame and sides. From there I built 2 sets of legs. Basically it is 2 C-channels that run about 6' into the box. Each C-channel had 2 legs welded to it and the C-Channels were then welded to the existing trailer. From there I welded up a headache rack to protect the back window, a beavertail support to hook the ramp on to and a 2 part ramp that is a total of 5' long (If I were to do it again I would make a 1 piece 8' ramp - the 10' is nice but putting it together is a bit of a pain). The deck can be put in the truck with 2 able-bodied adults with a little work, but with 3 or 4 it is a snap. Chain and turnbuckles to factory hold downs in the bed keep it in place (originally I used camper tie downs on the outside but they got too beat up in the winter with salt). After 2 years I upgraded to Superclamps - definitely worth the ridiculous amount of money they charge to not have to climb up there and mess around with tie downs.
Some pictures are below.
This is a bad picture of the leg, but you can sort of make out the C-Channel at the top. (I would make the feet bigger next time around - I have to put it on plywood plates to keep it from falling into the grooves on the bed).
The ramp uses an old truck box liner I bought for $25 off craigslist. And bolts together with 1/4" steel plates and wing nuts.
One of the first times we used it. Covers don't work real well up there - they tend to rip, never did that again. They stay clean pretty clean on their own though.
I have a pretty complete parts list for the aluminum that I used still. Let me know if you are interested and I will dig it up quick. Without any super discounts I think I paid somewhere around $500-600 for the aluminum. The ramp is the most expensive and time consuming part.