As I believe was mentioned before, it depends on the backspacing.
If the wheel's backspacing is incorrect, the tire will rub the frame (too much backspacing). If there's too little backspacing, you're changing your track width (pushing the tires further apart), and during cornering, can rub the upper fender on the tire.
I've seen pictures of race cars with 7 and 8 inch wide wheels, so I don't think your going to have any clearance problems with the sixes.
But, just to be safe, adding 1/4 inch to the backspacing of the 5.5 inch rims should center it up to where it "should" be.