I think you will have to make your own.
Try to find a 57-72 Ford truck in short box with a wood bed. You need the length measurement, cause I don't remember what it is, I think it is 74 inches. Then you can go to LMC at this link:http://www.lmctruck.com/icatalog/fb/full.aspx?Page=26
and figure out what you will need to build one.
Are you going to replace the metal floor with wood? If so, get marine grade plywood with an "A" finish on one side, and seal it with ?, I just used linseed oil, 6 coats in 7 days at 90* temps, the bed lasted the 15 years I had the truck and probably is still in good shape. If you build your own with plywood you can space the metal rails at what ever width you want to make it look like it is factory. You will need to cope the pluwood slats so the rails will set down to the same height as the wood.
I should add the first truck I replaced the bed wood on, was a 66 Ford Stepside and have done 4 since then. I never thought about doing it to a Ranger, I think it would be real trick.
You should also consider the weight gain. You will need to have cross bracing to support the wood/rails. I would suggest the use of SS or for a real nice touch, you can have the rails bent from brass and use polished brass carriage bolts, then coat it all with a clear acrylic coat.