Most forklift trucks are rated to lift X lbs at Y load center. In the USA it will normally be let's say 5,000lbs at 24" load center with the other proviso of to a certain lift height. The weight is calculated from the weight of the counterbalance and the hydraulic setup plus the chassis size. The load center is usually half of the load it will carry in this case a 48" Deep pallet. The lift height is governed by excess height and also the lilt mechanism. In the US the higher the lift the tilt is governed to be reduced by half. Now going along with those common sense ideas if you start side shifting or having a truck that emulates a side shifter or shifts its points of balance whilst turning then you had better be sure to reduce the load high up in the air. Otherwise it will act as a torque lever and tip the whole truck over. If you had a truck straight up the rack with 4 wheels it really wouldn't matter how high the mast was as long as it was plumb and straight. As soon as you turned a wheel though it would be all over, and you do get paid to do that in forklifts.