Don't feel bad about your draft arm as I've broken six of them in 1600 hours on my 3720. Warranty covered the first one and we've bought a couple spares since. Also have welded them when they broke in the hole such as yours with gussets on top and bottom both. Broke one last Sunday ahead of the hole by where the sway bar attaches. That was the first one of the welded ones that broke. 85% new break and 15% previous stress cracking.Load on it at the time of breakage was 5 foot snowblower pushing snow with the loader,not even blowing at the time. I have broken them with a tr3 rake on as that is a heavy unit (6') and my 5.5' rototiller. Bitched to John Deere dealer that they are not heavy made enough and his answer was must be using it to hard. I bought the tractor to do landscaping work with not to sit on the lawn and look pretty.I don't baby it or do I abuse it but I feel the arm design should be more like the old 4020 or 4430 which does not have a hole drilled directly through the arm but attaches to a casting with the arm forged below it.
Seems like the only weak place on the whole tractor/loader setup in my experience. Unit is a 2006 and I will check on the adjustable arms at my dealer when I run out of room to weld on these.