The ends of the drawbar that you made of 3/8" should be strong enough. I make all of my 3pt top link tower ends from 2" x 3/8" flat bar. However I suggest you use an H pattern with the cross piece welded to the tabs and used to cap the end of the drawbar. Weld the tabs and cross piece first, forming an H, then weld that assembly to the drawbar. That is how all my towers are done and I think it keeps the ends of the tabs from bending and is overall a lot stronger. I usually leave no more than 3" of tab sticking out. It seems to work excellently for top links and should work fine here too.
I was a little surprised to see them use 3" x 4.10 bar channel for the tongue. I really think it's a little too light for 4000 lbs backing up where stress and forces are greatest. 2000 lbs yes, 4000 lbs no. I probably would have gone with 2" x 1/4" or 2.5" x 1/4" square tubing or even a couple pieces of bar channel on edge in a V configuration (which would be much more stable). The professionally designed dump trailer plans I have use 5" x 6.70 bar channel which is over 50% stronger.
You are right about the tractor end of the tongue being a very poor design made even worse by their welding method. That end needs to be an H design like I mentioned above.
I have mods similar to yours that I will be doing once I find the time. As I have said before, these are estate wagons not construction dump wagons and should be treated accordingly. However, they are priced such that with a little effort they can be made into real work horses.
