James,

It's relatively easy - especially within the context of a slow-build ES :-).  Just copy the cutouts on the inner ribs to the outermost rib in the existing fuel tank.  The vent tube gets cut shorter so it vents to the new outermost fuel bay.  Then you should move the fuel filler out one bay.  The fuel level sensor holes need to be moved so that the fuel level tube can extend to the outer bay.  On the other hand, I didn't move the fuel filler, nor did I modify the fuel level sensor on mine.  That left a little bit of air space in the outer bay, which actually is sort of nice, as it gives volume for fuel expansion and reduces the amount of fuel that can drip from the vents.  And my fuel level gage only reads to 38 gallons.  To me that is no problem, as knowing the exact fuel quantity between 38 and 45 gallons is not particularly important.  Sure, you can increase fuel capacity beyond the 90+ gallons given by one extra bay, but then you are into a fair amount of work.  You pays your money and takes your choice.  At a LOP, modest power setting I can get an absolute endurance of close to 8 hours and that's been enough for me and my occasional 6 hour trip.

Gary Casey