[Mr Gazza]

I have often thought it should be possible to remove the airbox and fit K&Ns or something similar, then move the battery and other gubbins further forward, so as to be below the frame rails.

Then you could weld a plate across the huge hollow in the bottom of the tank and gain the best part of a gallon capacity....Obviously piercing the hollow in a few places before closing off.

A bit more than a couple of hours work for sure! and no going back either, unless you have a spare tank.
Much cheaper than a custom tank though, for the price of a bit of inventive bracketry and a few cables.

I agree with Suneye about range. I can go 120 ish miles if I bimble about the local lanes, but if I can get into 6th and go somewhere, It will take me 160 or more miles.