That should be plenty. You might consider going the moto ramp or Ikea bed slat route for lighter weight and to make it easier to remove. Most of the outfitters use a three panel configuration with each end resting on 1 1/2" angle iron rails. I did the same with a $100 three panel aluminum atv ramp from Harbor freight. I just drilled out the hinges and cut each one to length, glued and screwed 1/4 ply ( I'd probably use 1/2" if I did it again) and covered them in indoor/outdoor carpet. Each panel weighs 15lbs and the original ramp was rated at 1500 lbs, so they are ridiculously strong. You could set something like that right on the top edge of your 2x6 ledgers that you have on the wall. In my van, the panels fit very snugly in the rails and I've never had any problem with flexing or squeaking. To secure them when I'm driving, I run a long tie down strap over the end of the whole bed and down through a couple of the D-rings.
Harbor Freight ramps:
http://www.harborfreight.com/super-wide-tri-fold-loading-ramp-90018.html
My three panel bed:
http://www.fordtransitusaforum.com/...other-minimalist-conversion-4.html#post140802
JP
Harbor Freight ramps:
http://www.harborfreight.com/super-wide-tri-fold-loading-ramp-90018.html
My three panel bed:
http://www.fordtransitusaforum.com/...other-minimalist-conversion-4.html#post140802
JP