As far as i know there is only the three options on the p38, spare wheel well, luggage area or have a split petrol tank that holds about 5 gallon of petrol in one part and lpg in the other (iv`e seen shoguns with this systems fitted). I had a clasic with the twin torpedo tanks underneath both sides and it was great, i didn`t have any clearance issues but there isn`t enough room under the p38 for them to fit. I have the tank in the spare wheel well and carry a can of tyre weld when i change my tyres i will have them filled with with an anti puncture fluid.
Mark