Just gear it similar to how you would gear a T4, a 6.5 would be a good motor, if you are going to race it i would suggest a 7.5 in a RWD, A 6.5 can be a bit to much motor if the track gets a bit slick, back in the day most people used to gear a 12 turn brushed mod motor around 18/87 in T3`s, so use that as a base point for a 7.5 turn motor and go from there.