I ended up having North Hollywood Speedometer and Clock make the cable for me.
The transmission side is the same as a 5.0 Mustang manual transmission. The driven gear will have a 1/2" long guide pin on the end. I think on the automatics the pin is about 1" long. The longer pin will not fit the Tremec.
I had the speedometer end made the same as stock.
Bring a firewall grommet along and have them install it before putting the ends on.
You're running 3.73:1 gears right? If so, and your tire is about 26" tall (255/60/15 BFG for me) then get a 18 tooth driven gear. It's pretty darn close to perfect.
I originally bought Forte's cable but it was 7" too short.
Forte's also made an adapter that bolted onto the Tremec and allowed the use of your original cable. It costs more than having a custom cable made that fits better.
The cable cost me around $35 minus the driven gear. From Ford they run around $12. The Ford P/N is CODD-17271-B.
If your old cable is still good the shop can just put on the Ford end. Be sure to allow a little extra length. It's a lot easier to route a little extra than stretching a little extra!
If you can't find someplace to do that for you let me know. North Hollywood Speedometer is only 15 minutes from me.
I only have one speedo cable firewall hole. I believe it's sort of under and to the left of the brake booster.
Click here to see see my car and hear 5-speeds. http://www.geocities.com/casanoc