Normally fans are not custom built by a company that does not build fans. The concept of a centrifugal fan is not particularly complicated, but it is difficult to achieve target flow rate, pressure, and efficiency if your company does not do it on a daily basis. If you are not able to purchase a fan, another option is to copy someone else's simple straight blade design with known fan curves.
Answering your specific question: The top design with the two inch gap on the left is better at handling trash (foreign material in the air stream). The bottom design with the small gap on the right will have a better efficiency because it wont recirculate as much air around the housing. The style of the fan (blade shape) is really a minimal problem compared to knowing the flow rate, and pressure required. This NREL article has some good information on centrifugal fan blade shape, but it basically simplifies to particulate handing reliability vs efficiency.
For clarification of the application (for myself) I attached some references below for a "continuous moving fire zig-zag brick kiln"
Other than coal fired brick industry rules-of-thumb (unknown to me), here are some ways of estimating specifications. This is potentially how the engineers sized those 52 inch diameter fans above.
I would estimate the airflow requirements based on stoichiometric fuel combustion and excess oxygen requirements. If you know how much fuel is added and how much excess O2 is needed, engineers can calculate the flue exhaust flow rate that this fan will need to provide.
Temperature of the flue of the current operation should be known, but note that forced draft modification may change this temperature. Temperature of the air affects the density of the air. The lower the density, the larger the fan needs to be to transfer the same mass flow.
If the new zig-zag stacking configuration has been used with natural draft, you can estimate the minimum pressure requirements by measuring the pressure at the base of the stack compared to atmospheric; and add in the buoyancy pressure of the hot flue. This is a difficult one to estimate because the flow requirements and duct geometry are unknown.
I would recommend over-sizing the fan and motor by at least 50% (which the engineers have likely already done), then when commissioning, adjust pulley diameters to reduce the speed of the fan to the optimum operating flow rate. This also gives the ability to adjust the pressure/flow if other brick packing and temperature configurations are used in the future. A variable frequency drive, makes this even easier if it is available. I do not recommend using a damper as it greatly reduces the efficiency of the system. Note that if the electric motor is has less than 40% of its rated amps after optimization, it should be replaced with a smaller motor to allow the motor to be fully loaded and operate more efficiently.
Once the engineers have all of that information they just need to select a fan that meets the requirements of airflow and pressure for a given temperature. There are lots of good fan manufactures in every country and they will produce a fan that meets your needs cheaper than you can design and build it. Alibaba is a good resource if you cant find any.
As side note, the forced draft will greatly change the heating and cooling rate of the bricks. The airflow may need to be reduced from optimum fuel use in order to reduce cracking. Also forced draft may structurally load areas of the kilns with vacuum pressure that may not have had any load in the past (baffles, doors, walls, roof, etc). Even 500 pascals can cause significant loads over large surfaces. It would be a good idea to have the engineers evaluate that scenario before commissioning. Also, unintended air (tramp air) may leak through cracks and access doors creating cold zones that were not a problem in the past. Make sure everything is well sealed.