Hey Pete I love these 'build it yourself projects' as you get alot of satisfaction from the end result.
However these rallye tanks dont make sense to me. (unless you're entering a rally) If you need alot of fuel you obviously plan to ride along way, if you ride along way you will need alot of gear which will also be mounted on the back. All this weight on the back makes the bike difficult to ride in tricky situations and all but rules out sand due to the lightness of the front end.
I am also looking for an answer to carrying more fuel after a recent 9000km trip to the Territory from the east coast. I carried enough fuel on the back at times to do over 650km but narly creek crossings and soft sand were a real challenge. I am keen to do a Simpson desert crossing unsupported but there is no way I could do it with all the fuel behind me.
IMHO the only option is the Safari tanks to transfer enough weight forward to allow the weight distribution with camping gear to be somewhere near neutral. I am also concerned about Safari tanks but there seems to be no alternative.