The front/rear position difference of the preload nuts is normal, they are all like that. The front will definitely crank down 20+ turns to raise the front the needed 2.5". The rear will be close but I think you should be OK.
20 turns is a about 1.5 inches. You should have about that much left, maybe a little more. You will need to move the crossovers down 10 turns first.
It will be a bear and you will probably need a shock wrench to do it. Here is how I do it but I am not as pre-loaded as you will be with all that weight...
- Completely unload the suspension.
- Use a blunt drift and loosen the upper preload nut (and upper crossover nut if adjusting the crossover). Be careful to not turn the lower crossover nut while loosening so you don't get out of sync with the other side.
- Lower the bottom crossover nut 10 turns and hand tighten the upper jam nut against it so it doesn't move accidently when you are adjusting the main preload.
- Grab the lower spring and the lower crossover nut and start cranking. Turn the entire assembly of springs, separator and adjusting nut all at once. You "might" be able to do it by hand but it will get harder as you go. Some dry lube on the lower spring perch helps (don't use WD40 or anything wet or it will just attract dirt). Crank 20 turns.
- Repeat on 3 other corners but don't tighten the jam nuts just yet.
- Roll and settle suspension and measure. Repeat if necessary taking note how much it raised and make subsequent adjustments accordingly.
- Tighten all jam nuts, go for ride., ENJOY.
If you run out of threads on the rear, you will need to stop and set the front based on the back. It is perfectly acceptable (and some riders even prefer) for the front to be up to 1/2" higher than the rear. Supposed to help with handling although I have tried it and don't notice any difference.