It's always possible that the long range tank option that you are after is not a de Havilland modification, and therefore the technical data may not be held by DH Support Ltd.
Browsing through the online CAA Airworthiness Approval Note database, Tiger Moth G-AHVV was/is fitted with a long range tank under CAA AAN No 27333. A 10 gallon aux tank as per mod BAS/082/006 is used, together with a Christen Wobble Pump for fuel transfer.
Also in the AAN Database is a reference to the use of a Rollason modification to provide additional fuel in G-ASKP at some time in its past, using the fuel tank designed for the Super Tiger. AAN 25420 refers.
Is there a particular aircraft that you are using as your baseline? If so, it might be worth contacting the owner to ask about the modification reference.
Best wishes
KZ8