This thread is brilliant-nice work on the parts bin engineering and simplified fab. I'm also confident your 3/8" angle iron should suffice given the application. Angle iron is usually made from hot rolled carbon steel or something similar and most carbon steels have great fatigue life and yield/ultimate strength. The suggestion above to check for cracks using either dye pen or mag particle are solid processes and are used every day in aerospace for the same reasons. Any competent machine shop should be able to do them for a reasonable cost.
After you get past a shakedown phase and get it dialed in, it would be supremely cool if you could record/report those findings either in this thread or a new one. I couldn't find any specs for 60-0 braking tests for the stock Dusters of that vintage with a quick search, but I bet they're out there somewhere on the interwebs, and I'm betting it's somewhere around 140-160 ft...