Picked up a couple of NOS brake cables from Mike Borth (
http://www.borthautoparts.com), GM part no. 592283. Thanks to Alex for the part number!
They are about 1 inch longer than the original C15 ones, but have the same ends and should work just fine. And they look correct too! Mike may have a couple left, if anybody else needs them.
Thanks for all your help folks.
Owen.