I think the '60-66 arms are the same, but I cant be 100% sure. I do know the lower spring mounts on that vintage are interchangeable with 67-72 and are sronger (double shear).
Read my thread:
http://67-72chevytrucks.com/vboard/s...=501676&page=2. Starting at post #39 I did a coil conversion and it might have the info you are looking for.
It rides way better with the coils and with the trailing arm mount flipped it hooks up and puts the power to the ground real well!