Hi again Claudio. IF you do as Ron says, you can find examples of the method ron and I speak of. but, just in case it can help, i'll try to describe it in outline form.
Refer to the first pic. Toy see those two areas either side of the raised full depth center bore wall? Those two areas where the wall of the bore is cut down, provide a huge flow path for the Idle Air Control air to enter. We know that some IAC's stay open longer or more open than absolutely needed to support idle RPM's. Like the stock PCV system, this "extra air" from the open IAC valve, acts to rob manifold vacuum- and acts as if it was a vacuum leak. The goal here- is to use JB epoxy or similar, to fill in the low spots behind the cuts in the bore wall. IT's important to do this- not only to reduce airflow thru the IAC valve into the main bore. But also to replicate a full bore wall as much as possible. "cause those notches in the throttle bore, will disturb the waveform the Groove will generate. Your Groove, will be immediately upstream from these bore notches. It's therefore good to fill those notches, like constructing two "dams", and giving one or two "air spillways" for IAC airflow. The "spillway(s) you want to be as close to the throttle shaft as possible- as far away from the Groove centerline as can be done.
To make the dams to fill those notches get a toilet paper center tube or similar. Cut it down so that you can place it in the bore up against those notches on the left. Open the plate, and have the plate hold it inn place. When it fits well, it will be the form to backfill the low areas with epoxy. "This tube will give shape of the bore, to put the epoxy against. When the paper tube fits OK, remove it and use a 1/8" drill bit or etc. to rough up the aluminum in the low areas, to give the epoxy a better grip. Get some clear box tape, and tape over the side of the tube facing the bore wall and notches. JB Weld won't stick to clear box tape..
Look in Service Bulletins, look at anything re: IAC mods. I've posted w/ pics on several examples, but they are scattered in various Groove reports. I Think I would fill the top notch fully, up to the TB mounting surface, make it flush with the bottom of the casting, and extending back about halfway to the rear, towards the IAC itself. The lower notch i'd backfill flush to the casting edge, and extend the fill to or close to where that square well is.
IT would likely help, to make another cardboard taped form, inside that square well that leads to the IAC itself. This to make a walled off area to fill in for both notches. JB will want to flow at first, especially in large amounts as it will take to fill these areas. So having the cardboard forms gives control of it. However far back from the main bore you go w/ the epoxy, be sure to slightly overfill above the casting flat surface. Then when it's all set up, you can sand it flat after removing the "forms". You want to have a uniform flat surface for the TB mounting gasket to seal against, otherwise, air will leak into the main bore.
The last step after sanding flush the setup epoxy- will be to cut one or two channels in the epoxy leading to the outboard edge of the throttle shaft. The top notch has a good side contour that leads right to the shaft end. The bottom notch is kinda weird with that rounded shape. So I'd cut the top channel "spillway" along the curved edge of the notch, in the epoxy, with a small drill bit, to connect the square IAC port to the end of the throttle shaft. Make the channel about 1/8" wide. A little wider if you don't cut one at the lower side.
Be sure also, Claudio, to pull the IAC and get it and the whole well it's in totally clean! VERY important.
I hope this makes sense- it's awkward to write all that I see in this TB. After doing this kinda thing maybe 2 dozen times, it's second nature to me.
Tracy G