Thanks Woodworks. I had planned to use something similar to the Kolpin fuel containers. There is a company in the US that makes something virtually identical to the Kolpin's. T
he only difference is they dont leak and the handle and cap is in a better place.
My plan was to then fit an ALU plate to the rear racks and then bolt the tanks on either side and then place the ortlieb panniers over them. I have a trip coming up in a few weeks and so I ran out of time so opted for a $10 5 litre plastic fuel can strapped onto the rear rack. Gives me an extra 80 miles so in essence I have a 300 mile total range which is more than enough for my needs in Western and Eastern Europe.