904 transmission shift issues...

About the kick-down:
It is best to consult a service manual and understand the correct adjustments, they change from year to year with linkage styles. It would help to know the year and model.

To get it to work the throttle bellcrank radius of the original carb, and Holley should be made to match with an adapter at the Holley 390 if needed. Then revised carb throttle location, may require a length change, of the throttle rod.

There is often an adjustment of the lower trans rod, using a jig holes in the kick-down bellcrank bracket. Then the upper transmission throttle rod, is adjusted to spec at curb throttle. Typically by pulling the rod forward, then extending by 4 turns. This varies by linkage style!!!

Danger: Transmission damage can occur with incorrect transmission rod adjustment!

About the throttle linkage:
It needs to have enough slack to insure carb can return to curb idle plus perhaps 1/8". It also needs to open the throttle fully when accelerator is fully pressed. It should operate without binding, and return quickly. The kick-down may need to be disconnected, for this test with engine "off". Reconnect kick-down.