I was thinking swapping to 5s and taming your driving skills. Your unloaded RPM's are 37740. This is a touch out of that efficiency range of 28-35k rpms. Dropping to 5s will put you are 31450 rpms which should be optimal.
When calculating speed with Brian's awesome calculator, try adding 1" (At least) of tire ballooning. My 2 truggies are geared for 34-36mph and they both are going over 40mph.