you simply have the wrong offset.
The LS uses a 60mm offset stock - meaning that the mating surface of the hub and wheel are 60mm of-center toward the front of the rim.
http://www.tirerack.com/wheels/tech/...jsp?techid=101
there is very little you can do except get wheel with the correct offset.
With the spacers - you have already moved the weight of the wheel farther out on the suspension - which with the additional weight of the 20" is not a good idea.
Going with longer studs will make the rim lug centric instead of hub centric like the stock rims are.