Sounds like you have this gear:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXEZ09 It will not work in your car, you're going to need to buy a different brand and or style of gear.
If you're going for a FDR of 3.5 means you have a 53t pinion, since it is obvious you have never ran the car before how do you know 3.5 FDR will get you were you need? You will once again run into having to change a pinion or gear. You should go to your LHS and stock up on a couple gear sizes I would start with buying a 88t,90t, and a 92t spur along with your various size pinions you should have. (This is if you were going to be running 17.5 all of the time.) Gears are very cheap you should always have at least an extra on hand at all times.