I wish that I could give you a simple answer to your question, the design probably is good, but it is too close to be sure with the information you have provided.
 
First, power requirements:  I tried pitched-blade turbines but the predicted power requirement was about 70 hp, which will not work with a 40 hp motor.  Then I tried three-blade, hydrofoil impellers and estimated about 27 hp, which is good.  Using the 40 hp motor and 25 rpm, the upper shaft needs to be at least 5-inch diameter, so 5.5-inch diameter is good, especially for operating at a changing liquid level.  If the impellers are hydrofoils, the design looks good for mixing intensity with moderate process results.
 
The potential problem is with the shaft critical speed (natural frequency).  You did not tell me where the impellers are located on the shaft.  I don't have enough information about the impeller locations and weights to accurately predict the critical speed for the shaft.  The operating speed of 25 rpm is very near my estimated critical speed.  The fact that the shaft diameter is reduced below the upper impeller indicates that the design is very near the critical speed and the shaft and impeller weights are critical.  The detail of the mixer design means that the equipment designer probably took into account these factors, but the design is too close for certainty.
 
Finally, you will need a strong support for the mixer drive.  To handle the dynamic load of the mixer, the design torque for the support should be 252,000 in-lbs and the design bending moment should be 330,000 in-lbs.
 
With these qualifications and assumptions, the design should be correct.

The answers by this expert are based on the best available interpretation of the information provided.  The consequences of the application of this information are the responsibility of the user.  If clarification is needed, please submit a further question.