new to me 68 Barracuda Conv, sluggish 318

to adjust that KD;
The carb has to be at WOT.
Then the little Throttle Pressure lever at the trans has to be all the way to the rear. then you take up the slack until the slot in that upper rod is all the way forward against the carb stud.

If this still produces very early upshifts, then there may be one of several things at play here;
1) the apply-ratio is wrong. That is to say, the carb stud is in the wrong place. or
2) the lever down at the trans is the wrong ratio, or
3) the throttle-lever is not engaging the throttle valve, or
4) there is an imbalance in the transmission's internal pressures,specifically between the governor, and the total of; line pressure plus throttle pressure. The usual reason for that, is because the rearend gear ratio has been changed without a corresponding change in governor pressure.
This can be fixed.

The plastic push-pin inserted into the slot, is telling me that the pressures are not right.

Where the spring is hooked to, for the most part, does not matter, so long as it does not tend to cause the throttle to hang open at WOT. After the the linkage is adjusted as above, the rest-position of the slot , on a performance combo is relatively unimportant. The weight of the linkage sometimes tends to push the TP lever backwards which on a grocery-getter can make the light-throttle shifts harsh..... which tends to be annoying. So then you can use that spring to pull that linkage up off the lower bellcrank and that solves that.

If it was my car,tho, I would flip that slotted linkrod so that the slot was more parallel to the road.
I also do not like where your throttle return spring is anchored. Sitting up high like that, puts a lot of force on the primary throttle shaft, which will eventually wear out the bushings.... if any. If no bushings, the the baseplate will end up worn out. The spring should be down where the accelerator cable hooks to, and there should be a secondary larger spring around it, a backup in case the primary rubs thru at the hook. Each of them should individually be able to close the throttle from WOT.
Furthermore, your return spring looks generic and completely not right for this application.
And finally; taping the wires together is a bad idea. This tends to cause induction firing in adjacent wires. Mostly you can get away with this, because only on #5 and #7, do two adjacent cylinders on the same side fire back to back. So those two in particular, need to be separated by about .5 inch or more. Crossing over at 90ish degrees is ok. In your pic, you can see them running side by side all the way from cap to past the valve cover.