I helped do an SAS on my friends 87 4Runner. He ordered everything from AP - 4" SAS and rear kit. He called to complain that it sat butt-low. They told him he should be running 5" rear springs to level it. He asked why the didn't tell him that when ordering - they said they ALWAYS tell people to run 1" higher rear springs than front for 4Runners.

The PO of my 88 had the same issue - ordered a 4" kit from AP and it sat butt low. Not sure how, but he was able to get them to swap the 4" they sent for 5" rear springs...

You could try to get AP to swap the 3" for the 4" rears, but I don't know how far you'll get. Your plan of adding more front weight and removing rear will likely be enough to level the the truck.

bkg