For what it’s worth, Addco and Hellwig still offer rear sway bars for stock 63-72 Chevy trucks with coils or leaf springs. The Hellwig bars are bigger and cost about twice as much as the comparable Addco kit. These are all rear sway bars:
Addco #951 leaf springs, 7/8” bar
Addco #954 coil springs, 7/8” bar
Hellwig #7401 coil springs, 1 1/8” bar
Hellwig #7402 leaf springs, 1 1/8” bar
Here’s one outfit that sells them. There may be others with lower pricing:
http://www.suspensionconnection.com/...y_vehicle.html