the 63 are a thick leaf design, which yea they flex good but ride rough, to get good flex and a nice ride you need about 6 thin leaves, for the back of a 4runner i would get some 2wd F150 from say 88-about 93 year model, and just bolt them up with no block,
another thing is if that pic of the shackle is at normal ride height, the shackle is bottoming out you need another inch of shackle length
