Your 400,000 cp semi-solid will be very difficult to mix initially, with any type of impeller. A variable speed drive may be necessary to start and operate initially with such a viscous (semi-solid) fluid.  Even at 5,000 cp, the mixing will be somewhat difficult.
 
1. A 4-blade, 45-deg., pitched-blade impeller is suitable for this application.

2. A single impeller should be adequate with the vessel being shorter that it is wide or long.

3. The shape of the tank eliminates high-viscosity, close-clearance impellers as an option.

4. A 30-inch diameter impeller operating at 45 rpm should give adequate mixing when the viscosity has been reduced.

5. The motor should be either 15 hp, allow for some uncertainty in your viscosity and how it behaves at the starting temperature.  A variable speed drive is recommended.

6. You can view a picture of a typical pitched-blade impeller here.
 
Purchasing your mixer from a mixing equipment manufacturer, not a local fabrication shop, is strongly recommended.  The design of the mixer shaft, impeller blades, drive selection, and supports must be mechanically designed to be safe and successful.

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.