I have always been told that to put and 8 inch tire on a 6 inch rim is inviting disaster.
I think you run a very high risk of rolling the tire right off the rim.
My memory is not what it used to be. Changing tire sizes from original on the diesel also changes speedo readout and tranny shift patterns. I put an oversize set on my silverado and took the tires back to sears after about 3 weeks and went back to stock because of the different shifting, the different "feel" from the engine and the fact that the tires felt "squirrely"