Bisulphite/sulphite can be formed by contacting the SO2 stream with caustic in a packed tower. This is typically done after the gas stream is quenched. By maximising the SO2 strength ~18%, you will maximize your solution strength. The supplier of your sulphur burning system will likely be able to size and supply the absorption system as well.
In this system, you should look at controls and interlocks to minimze any sulfur carry over. This can occur if insufficient combustion air is added to the combustion chamber. A UV detector is typically used in the hot gas stream to detect any burning sulphur particles and a turbity sensor can be used on the effluent from the primary quench tower as it will become milky with sulphur carry-over.