the 3/s used DOHC for its entire life except for base model stealths and 97-99 base model 3000s. doing a rwd DOHC will be slightly difficult since there are no bolt on RWD configuration water crossovers for the DOHC heads. the 3.5 crossover is slightly longer to compensate for the taller deck height. IIRC the SOHC uses a different bolt pattern. this could be fixed by hacking up an NA 3s crossover and a DOHC 74 crossover then welding. (personally this is why im going DOHC 74 in mine. everything is already setup for RWD)

the DOHC 6G74 found in the montero are NA heads from the 3s. personally i want to put a set of TTs on and see how they work out (i would have done it but at the time didnt have any 2G TT heads lying around).

as for advantage....im 99% sure even the NA DOHC heads flow better than the newer SOHC 24v heads. the TT dohc heads would be a significant improvement