If I had the kick down linkage installed correctly on a poorly tuned engine, was running at say 40 mph and floored it, but the engine didn't pickup and go, either due to not enough torque or hp, would it be possible the transmission would not kick into a lower gear?

Or do I just have a problem with how it is hookedup? (Cable btw not actually a linkage.)

it is not really a kickdown linkage...it is a throttle pressue linkage.

what year is the car and trans...

part throttle kickdown did not become available until a certain year...

it is a 65 dodge. With the 727 from 65.

I dropped a 5.9 magnum into the car and hooked up the factory 2000 durango cable and springs and such so it would run off the factory tb and actuate the kickdown lever/arm whatever on the transmission.

It doesn't feel like it is kicking down even when I floor it. But I can't be sure because of how dog slow the engine is. Can't hear an rpm change. So I am not sure.

If you're going 40, kick it down and it stays in third, the kickdown is not adjusted right or it's not working right for sure.

Older 727's when you rug it is would downshift all the way to first.
Part throttle kickdown (from 3 to 2) first showed up in /6 trannies in 1967.
LA got it around 70/71.
Big block 727 got it much later.
Make sure you adjust the rods/levers properly.
There is a difference of rods between 2bbl and 4bbl,and also when swapping from the original motor to the 5.9 you have now.

unfortunatly there is no rod or lever. I took the cable setup from the 2000 durango and ran it to the TB like they did for the durango.

I check that it moved it, but it doesn't pull it all the way. I was told that it was fine as long as it put pressure on it, full movement wasn't completly necessary. May have to go back and revisit that.

It needs to go almost all the way to the rear in order to downshift. The cable is adjustable at the carb bracket end.

The TP lever on the '65 transmission is longer than that used on most later units, if you can try to find the lever from the original Durango and it should work like the original.

I did take one off a van or something to use.
The two I took are the same length, and I don't have a good pic of the original.

here it is on the trans.

For those that care. It was in fact an adjustment issue.
The new cables have a built in adjuster, and I popped it lose and it fixed it.So now the transmission down shifts.
There was about a 1/4 inch to 1/2 inch of travel left in the lever that got sucked up by the adjuster.

Cool.