There is no harm in putting 5 forward transmission in a SOHC. 5th gear is for highway fuel efficiency, like on 4 forward, you achieve 100km/h on around 3300RPM in 4th gear as compared to 120Km/h on same rpms on 5th gear. After one hour of usage, you will reach 120Km in 5th gear while 100Km in 4th gear in almost same amount of fuel. Only the lever assembly console needs to be altered.