Just wrap the bar with a few layers of electrical tape at the point where the bushing will be fitted.

I think you will find that stiffening the rear bar by 2MM and leaving the front bar as is will give you a more neutral fell than if you were to stiffen both bars.

A few years ago I put Sportline bars on my 300E. I just this past weekend put the original front bar back on. It feels much better. I should, however, point out that I prefer a little oversteer rather than leaning toward understeer.

Good luck,