ideally these rear arms need complete make over using thicker material but keep same measurements so they look like for like....bl or leyland made on the cheap cheap 70`s teas up!
Yes i've seen cracked and broken trailing arms, who hasn't? But i've seen far more failures due primarily to corrosion than I have to overstrain and these seem to be, in the main, on track, or very hard road use cars, that have been polybushed throughout. I even took a busted (rotten) one off a 1500 auto recently, I can't think of a softer life than that in terms of strain! But rot is no respecter of model!
My own Carledo has probably some of the hardest worked trailing arms of any Dolomite anywhere, with 140odd horses on tap, frequent track work and, the REAL killer, lots of linelock controlled static burnouts on the dragstrip. The car only has Poly on the rear end of the trailing arms and the arms are the 73 originals (Ziebarted when new) I've not strengthened them and they are still in perfect shape.
So, once again, my advice is, if it ain't broke, don't fix it! Just check your arms now and again for rot! Getting them cleaned up and powder coated, as Tony is doing, is probably gonna go a long way to ensuring his arms have a long life. I also agree with cleverusername, if you stiffen that initial failure point, you don't cure the problem, you just move it somewhere else!