The plug that comes with the kits is way too big, something like .130 or .140 if I remember right. Like John said, you need something well under .100.

And you don't have to drill, tap, and buy expensive brass for this. A box of Dorman part number 555-115 3/8" core plugs will do. Just drill the port so that it fits snug. Even if it's not real tight, it can't go anywhere. Start in the .080 to .090 range. If the hole is too large and still slaps, run a screw into it and pull it with a claw hammer. If it is too small and flairs, you can pull it and replace it, or you can put some grease on the end of the next size drill bit to catch chips, drill it, and then flush it with some brake clean to catch any missed chips. (if you don't like that last part, just ignore it. I know it's not a good idea, but I have done it a bunch of times, so...) And you can leave the valve body hanging by the park rod if you want.

One more hint, jack the car up higher on the driver's side so that the fluid runs away from you as work from that side.

Once you get close, .002 or .003 will make a big difference. And with the temp swings of the fluid, you may not be able to get it perfect for every occasion. You may have to settle for a little slap at very low throttle or a little flair at WOT. Pick your preference.