Here is a good write up on an adjustable version of the trailing arms made at home:
http://67-72chevytrucks.com/vboard/s...d.php?t=321100
Here is a link to djracer's build for his truck. On page 3 that is a picture that you can see the stock arms in. He may have only welded the seam on the top and bottom, can't really tell in the pics.
http://67-72chevytrucks.com/vboard/s...d.php?t=151718
For reinforcement, I would weld the seams top and bottom or even weld plates to the top and bottom for the whole length of the arm.