Hope im on track

That opening can't be too big.... It allows for easy transfer of heat/smoke to the cooking chamber.... Too small and you overheat the FB and burn too much fuel.....
The end of the plate need a large opening also, and the stack needs to accommodate the flow also for the system to work...

The ideal set up for heat and air flow control, on the FB, would be something similar to the picture below....

With the inlet that "oversized", it will be difficult to control the inlet air flow.... The opening slit, when operating, will be very small and adjustments will be difficult at best.... I'm thinking of how you could make the change.... I'll be back.....





EDIT


It looks like most of the welds are "tacks" so to speak.... I would remove all the metal that is associated with the inlet controls.... sit and ponder what you want to do.... OR

I'm thinking what I'd do is..... cut it all out, weld on a new plate and add the new inlets.... RED LINE is where I would cut stuff out.....



Get the replacement piece of steel cut... make sure it will cover the cut out, or cut it out AFTER you get the steel.....

Then build the inlets on the new plate while it is on the bench and weld it to the FB.... have the lower and upper inlets already in place...