The 70-72 Dual Gate cable is 30" overall and 18" is the casing length. Regular floor shift cable is shorter (27 1/8" and 16 5/8"). They are available from Year One, Fusicks, etc.
Here's a tech site on (what seems to be) the 79 Dual Gate including installation:
http://hrst79.tripod.com/79hrst/id11.html
I'm not sure if that site is correct, though....does yours have the neutral safety switch in the shifter housing? 70-72 had the neutral safety switch in the normal location under the steering column, so wiring is an issue.
C.J.