I doubt you hurt the transmission, but you need the spring hooked up so that the throttle lever on the trans moves with the throttle on the carburetor like it should. If anything, you've got too much throttle pressure right now under some conditions, it might be affecting your lift-foot upshifts or upshifting at low speeds.
The single purpose of the slotted linkage and spring for the kickdown rod is safety, by the way. Early kickdown rods were connected (usually by ball/socket joint) directly to the carburetor throttle shaft. A limb or other debris passing under the car could pull the throttle wide open if it snagged the kickdown rod. Hence, the slot and spring.
D/W