This is on a home built 9 inch ford housing in a 66 Chevelle. Near as I can measure, I need to either cut 1/4 inch off one side or move the whole assembly sideways 1/8 inch to center it perfectly.

I know this does not seem like much but is important when fitting 10 inch rims with 295/50 tires.

When perfectly centered there is about 3/8 inch from the tire sidewall to the frame.

I suppose with the stock style control arms there is not much to be done.

With adjustable control arms top and bottom would it be fair to say that the housing can be centered perfectly by making one side a bit shorter and the other longer?

Would it be worth to fabricate a panhard bar?

If 1/4 inch is that critical a panhard bar just wont cut it. With a panhard bar there is still side to side movement as the body raises and lowers. A Watts Linkage http://en.wikipedia.org/wiki/Watt's_linkage will keep everything centered and still allow normal up and down travel.

Adjustable upper control arms will make it easy to center. :) Use ones from Edelbrock or Currie with their spherical Johnny Joints.