You cant get custom tanks made, well actually i think you can but unless you want to spend a good couple grand getting that done it wouldnt be an option.
You can get other tanks which fit as a donut in your spare wheel but you have a box there so that idea goes. I doubt you can put one underneath, unless you pay a bit of money and get the wagon tank and put it inside your fuel tank or somethign in those lines.. Best bet is to call GRA (Gas Research Australia) in Victoria and have a chat to them, their one of the best LPG Companies in Aus for installs/equip/performance on lpg.
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