John Kunkel has it right when it comes to the Lokar kick down cable. I installed a Lokar cable in my 64 440 Belvedere 18 years ago. I followed their install instructions, but was always dealing with a late 2-3 shift.
After reading some of his prior posts here, I finally did something about it earlier this year.
First I drilled a new hole to relocated the cable attaching point on the transmission lever inward about 1". I also added a return spring that I installed in the original hole that the cable previously attached to.
A big improvement was realized. I finally have a transmission that shifts correctly.

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