I have in the past shortened both inner and outer cable.I cut off one end of the outer cable
and duig out the old piece then taken an old pair of wire crimpers and ground them to make a cable crimper out of.
For thye inner cable it is not as easy. After you determin the proper length, before you cut the inner cable solder the cable on both sides of your cut mark. Cut off with a dremel or a cut-off wheel. To square the end is a little more difficult. I had a tool to do it with. Best I can say is get 2 pieces if thick steel and take a file on edge (to cut a "v" in both halfs) put one in vice and the other over the cable and smack with a hammer.
Thats the best I can say it is just easier to get the right cable