Rodney there will be solutions.
Where do you want the tank to sit?
1) taildragger style so you maintain your current rear box.
2) on the passenger seat in front of the rear box eg a Tourtank keg. No pillion with that option and access to electrics etc under the seat can be a hassle.
3) on the passenger seat but underneath, like a custom alloy flatbed style and using a different rear box eg a pelican case. Not elegant on a cruiser, but very efficient and practical. No pillion.
4) on the rear rack replacing the current box so long as the rack assembly is up to the load. Something like a JAZ tank could bolt on there pretty easily. Luggage space is lost of course.
5) somewhere else eg hidden in a pannier. Likely more complex and probably not the best option. Luggage space lost.
6) Frankentank enlargement modification to the front tank.
How do you want to plumb the system?
Options 1 and 5 above need a fuel pump. Options 2, 3 and 4 probably don't and should work with just gravity.
Are you willing to drill and bulkhead the front tank as Tom suggests above? That is a common approach. The plumbing is pretty simple after that: just supply and vent hoses, filter, drybreak and a tap.
Inserting a T-piece into a return line is possible, but may require a fuel pump.
A general guide to that process for an FJR:
http://www.fjr1300.info/howto/bulkhead.html
Specific location for where to drill will take an inspection of the underside of the front tank.
What kind of tank?
- keg
- plastic or alloy of the shelf JAZ/RCI
- custom alloy
Mounting?
That rear rack assembly looks strong. Finding additional mount points for a rail over or instead of the rear seat should not be too hard, but I don't know that model.