Aftermarket gauge quesiton

The times I've broken replacement speedo cables, it was either do to poor routing, (got hot and melted on headers)
or it got kinked when it was installed. The kinked ones squeak for a minute, then fail, almost immediately.

You might take a second look at how many turns it has to make between the instrument panel and the transmission...

I have no experience with any other aftermarket cables....

Yes you can, but it will take some ingenuity, and fabrication skills. One of the guys on here built a custom dash while keeping the stock speedo. He posted some excellent pictures of what he did to make it work.
Basically you would be working in reverse, but same idea. Do a search for it, sometime between July and last month. I'm sorry I can't remember who did it, but it came out incredible.

I did a custom gauge package for the '66 that replaced all the gauges that may also be helpful.
There are some pictures in this thread. I have more pictures of the backside, it it helps.

http://www.stangnet.com/mustang-forums/789682-new-gauge-cluster.html

BTW: If I was doing mine over, I would get the 4" speedo. It would take a bit of bezel trimming, but should look better than the 3-3/8" one I used.

Good luck!