I'll definitely investigate the Mulgo conversion, thanks! As we discussed previously, I've previously had the left hand one installed in a 300Tdi. The after market cap never sealed 100% and I always had a dusty run streak down the left hand side, hence the reluctance to cut filler holes. With a 2-way switchable transfer pump you would be able to keep the LHS tank full for a back up and just use the RHS and main in your fuel planning (110 liters?). Food for thought...

Drover, I'll look into the Long Ranger option as well. As KarlB said though, on a 90 the rear cross member is too close to the axle to fit standard replacement tanks. If they actually came up with an idea of how to make this work, they'll sell like hotcakes overseas!