You can forcefully set 40MHz channelization under 2.4 radios in the AP Group.
The point is however, except for channel 6, you are trying to operate outside
the IEEE defined 2.4G WiFi frequency ranges.
I don't know if you can force your clients to use 40MHz channelization for 2.4G,
but if so, you will likely see poor performance, (except on channel 6), like Munish
said.