FordFocusDanielSan,
Depends on who's aftermarket intake kit you are buying.
In fact, most of the intakes use a rubber elbow from the tubing to the t/b, so it might be impossible for you to do anything at all.
The MAC short ram for example has a 3" (I think) tube, with a rubber elbow that fit's the Zetec throttle body. So it's going to be difficult to put a reducer onto the rubber elbow. Same is probably also true for an AEM short ram and full length CAI.
Are there any intake kits that have a metal tube that is bent, or has an elbow built in, with a rubber sleeve to the t/b? I'm trying to remember is those Ractive kits are like that or not.
I know the Iceman kit of course has the "built in elbow" and uses a rubber sleeve to adapt to the regular t/b, and of course they have SPI kits with the proper reducer. If no one else has the answer you are looking for, I can measure the Iceman reducer for you next week when I get back to my store.
BTW, I have a Iceman SPI kit (Red) in stock that I would give you a good deal on to get rid of it
Shoot me an email if you are interested.