Do the 1275 auto's use much more fuel than a manual 1275?
I wouldn't think the work and expence was worth it just for the range you are going to get, unless you do lots of miles between fuel stations.
How are you going to plumb up the second tank, with a seperate filler? pump etc fuel gauge or are you just going tosypon it through the side tank? if you are just syphoning then if the top is higher than the other tank then it will never get full same goes if the bottom is lower than the side tank, you will never be able to empty it completly
Terry on the moke forum had an extra tank made up for the rear of Desert Storm much the same as an export moke tank he has a system of valves and a pump to pump from the rear to side tank and visa versa but it takes some planning to get the best range.