.....Chaz, not sure if your confussed on this or not but; a fish plate {they also call this diamond plating}is a plate that you put over an existing weld to re-inforce it. This is a piece of steel that is cut in a diamond like shape, or can be a square piece that you put on at a 45 degree angle to the frame. You would grind the weld that you are covering so the plate would lay flat on the frame. You don't want any 45 degree welds on this. Re-read what Classickustoms wrote in #4 suggestion in his post.........
Diamond plate is steel that has a diamond patern on it. This pattern keeps you from slipping if you walk on it when it gets wet. They use it on steps that are on 18 Wheeler Trucks {plus lots of other things}. Nothing to do with fish plating a frame OTHER than the fact that fish plating a frame can ALSO be called diamond plating a frame.....
Boxing plates are what go inside of your existing frame for support {this I know you know}......
I'm not sure if this was possibably confussing you or not.... just wanted to make sure..... bill