Just for the record the tire size (width) selected for a given counterbalanced lift truck is based on two factors 1.) the load capacity rating of the tire per the tire manufactuer, meaning when the tire is carrying a max capacity load the tire (axle) loading should not exceed this tire rating. 2.) to improve lift truck's lateral load capacity capabilty, by increasing the base of the stability triangle or tread width (measured from center to center of the front drive tires tread. As duodeluxe pointed out teh tires must be pressed on to assure the correct tread width is achieved and to insure that the inside of the tire does not extend to far inward to cause interference with the mast at it is tilted backards. Typical when tires are presson the max amount of tire ring over extending the wheel rim is 1" in one direction.
My reason for mentioning the last part is that "most" tires extend over the outside of the wheel and the inside is flush with the wheel. So when you remount the mast, becareful & watch for interfernece or pinch points between the mast & tires as you install the mast & then when the mast is back tilted.
Plus, it would be a good idea to verify what teh front tread width should be with the 6" wide tires & meaures what you have.
If you need adjustment the tires would need to be repressed by 1/2 of the difference of what you measured as tread width to what the spec sheets of teh truck states as the tread width.