you should get alloy upper rear a arms to avoid spur gear destruction
You will still have the same amount of flex in the rear end with alloy A arms. To see what I mean take off you rear tire on the gear side hold the rear end and twist.
You can lessen or leave the flex and avoid damage by doing one of these two things:
1 - Replace the rear end and lower arms with alloy, which stiffens the rear and lessens the flex and also takes most of the cash out of your pocket.
2 - Which is the lesser cost, and what I will do soon. Go ahead and let the rear flex, but install the alloy upright lengthening set and alloy rear raised uprights, this will lift the A arm rod out of the way not letting the flex cause rod/gear damage. FG part#'s - 6478,7478 for FGMT.
Now also to keep in mind that there is flex in both buggy's and monster vehicles but there is twice the flex in the monsters due to the size and tire weight and roll over potential. I found this out really fast in switching the monster tires to a buggy that I never had any flex problems before, and really quickly found I did with the larger tires on it. It also will depend on the motor mesh, this will depend on how close your large spur gear gets to the A arm shaft. If its far enough away it will take some real time flex to hit it, or it may never hit it at all.