OK. First thing. Do you know where you are in your vacuum modulator adjustment? The last remark I saw was that you were at 3 full turns counterclockwise. I don't have a large sample size as I stick to my own transmissions and an occasional friend's car or truck here and there. I can't recall ever having settled on an adjustment that was counterclockwise to the factory preset. In fact, some of the builders I know give it a full turn clockwise before it even goes in the car. If you know where you are, return it to the factory preset or even one full turn clockwise and go from there. If all of your adjustment is happening in an overall counterclockwise from the factory preset, it could explain the lack of results with it.
Second. I can assume from your last post that the kickdown/detent linkage is connected. (That detent can be modified to stay in a mostly depressed position, but let us go with the far more likely scenario where it is connected) The vacuum modulator can give you a softer/earlier shift at part throttle, if it is adjusted that way. However, the detent linkage will override the modulator when you have a more aggressive throttle position. Don't get caught up in this idea just yet. One thing at a time. I was just making an example of what can happen.
Lastly. Sometimes there are a couple of things you can see with the pan off when a shift kit is likely to have been installed. The detent cable modification would strongly suggest a shift kit install. If you see a doubled up steel plate in place forward of the valve body, this is pretty much a guarantee. It allows the direct clutch (3rd gear) to be "dual fed" to improve the holding power in 3rd gear. It's a great modification. There are others. I just think this is what I have seen the most.
Some of this may seem like foreign language now. It won't always be that way. The last 2 points are that we aren't out of answers yet. Stick with it. You can and will get this figured out!!!
I am a "back yard" mechanic and by no means an actual transmission guy. There are A LOT of folks on this site that are experts. They can feel free to comment or correct anything I said. We are all in this together. We are all still learning. Please, keep us posted on what you're doing and how the car is responding.