You did not tell us what you are using for suspension bars on the rear or if you have an anti-roll bar.
To me it sounds like you have several things out of whack as you call it. The fact it is driving straight is really mind boggling to me. I can only surmise that bars are creating a preload and different spring presures are counter acting it.
I have seen only two cars lift the right front and keep the left front down and on both cars I was the one doing experimenting with bar and spring pre-loads. Even those did lift the left front on first gear change but a little down track were lifting the right front. That was only during research and expermenting. They were not left to run like that. One car was a clutch car and I had a 120 # spring on the right and a 90# spring ont he right with far too much bar pre-load cranked in trying to solve a down track problem of it driving to the side under power. Turned out the whole problem was fornt suspension alignment and not originating from rear suspension at all. The other was a car that was driving to the right hard and I really cranked on the bars. It looked funny when it lifted the right front in 2nd. That one had a tire that was growing more on one side than it was the other side. Took forever to find that problem.
I suggest to begin with working out your spring rates and getting the right to left in the rear even if possible but at least within 70 lbs of each other. That is with zero pre-load in the bars. Make a pss and video it or have a good obser watching. Be prepared of the car to go either side or straight and at the first hint of driving to one side back off and back to the pits. If it is driving to the right you will need to increase pre-load to the right rear if it is driving to the left decrease the right rear pre-load.
If you have an anti-roll bar unhook it anytime you are making any rear adjustments and connect it back afterwards. I like them to have all bolts and pins not binding with all weights including driver in the car. For some reason Mustang factory style 4.links are different and the anti-roll bars need to be adjusted like David Wolfe says to do it.
Ed
Ed