Not a bad idea, Have a look on
http://www.gasure.co.uk/tanks.htm
45ltr zero degree tank (580mm x 200mm) specifically for the Type 25 (T3) to fit in place of the spare wheel (NO STOCK) at the mo but may be worth a call.
I did think of this one, The main drawback is the size. The tank will only hold about 35ltr so would be filling up every 150ish miles.
Ended up going for a 70ltr under the r&r.
K