6* should be plenty. as long as it's positive and not negative. positive lowers the pinion into the ground, negative raises the pinion into the body. you have the shims in right I take it?
Sounds to me like your new steering box setup might just have more leverage over the steering on the cruiser, making it difficult for the tires to force a return to center. My 4runner is kinda like that too. It wanders a bit, you get used to it. Toe in or out plays a big part in it all too, Allpro recommends my high steer to be set a quarter inch toe in. That sucks in my truck, I get bad death wobble, so I run my steering a half inch toe out and everything is good. I guess you just have to play with it.