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This step by step tech article was provided by Jefferson. Thank you Jeff.
Full Throttle - 99-04 throttle cable adjustment/fix
Introduction
There is no better feeling then when you drop a gear and put your foot to the floor, the engine growls fiercely, your Stang jumps forward and you're gone, leaving everyone and everything in your dust. But did you know that you could be doing this easier?
For whatever reason, the 99-04 Mustang (more noticeably in the 99-00 models) have about an inch and a quarter of slack in the throttle cable. This causes your gas pedal to sit lower than you break and clutch pedals and makes it more difficult to switch between pedals. Luckily for us, this is a very easy fix. (The process took me about 10 minutes!)
In this article we will fix this "problem" by using a spacer and some fasteners where the throttle cable connects to the gas pedal to reduce the slack. This will allow you to achieve a wide open throttle and raise your gas pedal to about even height of the brake and clutch pedals.
What you will need
1" hollow nylon spacer or some stiff plastic tubing 2 small hose clamps (for testing/temporary use, zip ties work great!) Utility Knife (Exacto Knife) Flat head screwdriver
I went and bought a 1 foot length of stiff plastic tubing as it can be easily cut to suit and you have some extra just incase. I ended up using about a length about one and a quarter inch (1 1/4") long to take up all the slack. The result is a very touchy gas pedal, the less slack taken up the more throttle play you will have. You may want to make a slight change to the length after you give this mod a try. Zip ties are useful because they are easy to put on and cut off, but I would use the hose clamps as a permanent solution for when you decide on a length because zip ties could slide/move.
What to do
First, make a slice along the length of one side of the spacer so you are able to slide it over the throttle cable. Next, squeeze yourself into the floor so you can see the top of your pedal, where the cable links to the pedal arm*. Lift the gas pedal, this should cause the slack in the cable to become very apparent. Now, slide the spacer over the cable. The spacer should now be butting up against the cable's end cap, the pedal arm and holding the cable out. Use you fasteners (whether you are using the zip ties or the hose clamps) and fasten one near each end of the spacer. Make sure everything is secure, and...
THAT'S IT!!!
* Alternatively, you can detach the throttle cable in the engine by pushing the plastic throttle body linkage forward then slide the cable out of its slot. This allows the cable drop about 8 inches at the gas pedal, where it is easier to work on. However, I prefer to work on the linkage while still attached as I am able to see how much slack is being taken up.
Your pedal should look about this height
Sit in your seat and rest your foot on the pedal, you should feel an immediate difference in height. Go for a spin, make sure you like the height and everything is good. If you used the zip ties you should now swap for the hose clamps because the ties could move/slide.
Printable Version
Full Throttle - 99-04 throttle cable adjustment/fix - 
. I ended up using about a length about one and a quarter inch (1 1/4") long to take up all the slack. The result is a very touchy gas pedal, the less slack taken up the more throttle play you will have. You may want to make a slight change to the length after you give this mod a try. Zip ties are useful because they are easy to put on and cut off, but I would use the hose clamps as a permanent solution for when you decide on a length because zip ties could slide/move. 
