I replacement one in a 66 vert about 5 years back. I purchased mine from Texas Mustang, and was very impressed with the quality of the repop. It seemed stronger than the original would have been new. The welds on the box look very nice as well.
Im not sure the gauge of metal, but they are much heavier and thicker than the other metals. I got mine from the mustang corral in Edwardsville Illinois. I think all of the repo boxes are heavier metal.
Mine were the "american" brand from my local Mustang parts house. Fit was reasonable though I had to take it apart to get it into my Vert. Things went back together well and seemed like a decent guage of metal for the application. the old ones were toast so not much comparison.
I got mine from NPD and was "american" brand as well. I only needed the top part that rusted out because of the floor. I separated the piece that I needed and welded the new piece to what was left of the original. It actually fit very nicely except for where it joined the front inner frame rail. I bridged the gap with a piece of the remaining box and it looked great. The guage of the steel is a little thicker and I agree, is one of the few repo pieces that fit nicely.