On the tweak issue--
I've found that the lower deck can tweak between the four screws for each bulkhead. Just loosen each of the four (w/o load on the chassis) and re-tighten. The shock towers can do the same. With the chassis on blocks, loosen both screws and re-tighten, making sure the shock mounts are equidistant from the set up board (use calipers).