Little help with kickdown on Super Six conv

Yes, you want the slot bottomed out on the throttle pin.

According to the FSM you should make the adjustments at the transmission and linkage points, however, a quick way to 'cheat' is to use a nut/bolt/washer in the slot to give a little more adjustment. I did it that way for years and it worked fine. It's a common mod.

The slot and spring are there for safety so if the kickdown rod gets jammed it won't hold the throttle all the way open.

Add the spring, as @RustyRatRod sad it's NOT good for your transmission without it. You need added pressure the second you crack the throttle plates. Additionally you need pressure release when you take your foot off the gas. Currently you aren't achieving either consistently without the spring.