Turning 6th gear into a cruising gear? Possible?

I do lots of woods riding, but getting to the woods I usually ride the bikes a long ways on a dirt road and would like to have 6th gear just as a "highway" gear for easy cruising without losing the power in all my other gears as I would if I went to a smaller rear sprocket or bigger front sprocket. Is this even possible without major headache? My 6th gears on both my bikes are so tight, I am always wishing I had 7 or 8 at that ratio. Any help would be greatly appreciated.