Actually in my experience M/T's have a stiffer sidewall than many. If you were unstable at speed I'd say that you were underinflated or undersized. Street/strip radials require some pretty specific chassis tuning to work real well. I suggest you stay away from them unless they are required to make a class. ET streets both biased and radial(see above comments) work pretty well with the biased a little more consistant and the radial a touch quicker.