Hi,
I’ve not had to do this for some years so my info might be outdated 
First off - the 40Mhz channel width and custom frequencies for that matter are TOTALLY wifi card dependant. So if the card EPROM does not indicate it as possible channels Mikrotik won’t be able to set to those channels.
In addition you likely need a ‘super-channel license’ as per http://forum.mikrotik.com/t/5-4ghz-superchannel-license-needed/15093/1
Regardt