I'd stick with the stock design and would try to find a good used trailing arm. No need to replace both if the other one is in good shape.
Its been my experience that when it comes to buying replacement parts it's always better to use OEM components if available. If you can't find them locally I am sure Tony or Rich can help:
http://4-5-6chevytrucks.com/http://stovebolt.com/RJ/I might also add, rusting trailing arms is pretty common, especially in rust belt states. Since the arms are made from two fabricated channels, fastened back to back, the mating surfaces are "ripe for rusting". My Dad had a '72 and the trailing arms actually rusted so bad that they folded.
Luckily it did this sitting in the driveway at my Grandpa's house.
