I am almost afraid to answer this question, not because of the information provided, but because of the missing information.
 
The impeller type is not specified, however, I quickly discovered that if I assumed the impellers were pitched-blade turbines that the power requirement would be in excess of 130 hp (not likely with a 40 hp motor).  If the impellers are narrow, three-blade hydrofoils the minimum power requirement is only 31 hp.
 
The real problem is in checking the mixer shaft design.  Insufficient information is provided to accurately estimate the length of the mixer shaft, the spacing between the impellers and the weight of the impellers, all of which influences the shaft design for both strength and natural frequency.  The variable liquid level implies that the hydraulic load factor for the impellers should be 3.0 or higher.  That load factor does require at least a 5.0-inch diameter shaft.  However, using a 5.0-inch shaft, instead of a 5.5-inch and 4.5-inch shaft, my estimated natural frequency (critical speed) is 19 rpm.  With the operating speed at 25 rpm, this large mixer must pass through the critical speed quickly enough to avoid serious mechanical damage, especially when operating entirely above the liquid level.  Most large mixers are designed to operate below the first natural frequency of the shaft.  This design may not meet that requirement.
 
With the design involving a stepped shaft (5.5-inch to 4.5-inch diameters), the assumption would be that the impeller weights and locations are such that the critical speed has been increased above 25 rpm (the critical speed should be above 32 rpm).  However, that much of an increase is unlikely based on experience, especially if the impellers are designed for a 3.0 hydraulic load factor.  Only a detailed mechanical analysis with information about the location and construction of the impellers can an accurate evaluation of the mixer design be done.  The most serious concern about this design is that the actual critical speed is above 19 rpm and too close to the operating speed of 25 rpm to avoid potentially dangerous operating conditions.


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.