I'm sure it can be done but it might leak on you.....Sorry I own a Harley but I still like the Harley humor.
Now.....Unless you are riding thru some desolate area or to Alaska I'm not sure the pay off validates all the work and aesthetically that tank will not look good on a VTX. One of my favorite things about the VTX is the tank and the accompanying headlight style.
Just be easier to stop at a gas station every 100 miles to rest your bones and arse.
....But it can be done. All you have to do is go down to the HD dealer and rent one from month to month and bam you got that extra 1.5 gallons, plus you get bags, cruise control, abs brakes, possibly a stereo and with navigation to boot. Instant gratification!!!