Found DORMAN Part # C92421 to match the GM parts catalog number 3955501 for $19.27
http://www.rockauto.com/catalog/catalog.php
http://www.rockauto.com/catalog/moreinfo.php?pk=358274
http://www.rockauto.com/catalog/x,ca...Cable%2BDORMAN
Just not a 100% on the application. Looks like the 68-70 coil application w/396 is the same as the 71-72 C20 127/133 trucks and the 133s had leafs only.
Something to consider at this point if a trans. swap is near term you might just want to clean and lube the stock cables if they are not stretched too much.
The image below has all three cable applications and lengths for you.