How reliable are aircraft cutaway drawings from Flightglobal?
These cutaways are just general representations of what structural components go into the aircraft; they are NOT engineering drawings nor should they be used as such.

In general the OEMs and the engineering departments for the airlines are pretty sensitive about releasing engineering data for proprietary reasons.  If you are working on a product closely with an OEM or an airline, they may be able to grant you a license to access such data, generally for a substantial fee. Or, if you have a client who will allow you access to the aircraft, you may be able to take engineering photos and/or make detailed measurements directly on the aircraft.