There are no 4-Valve Stelvios, so that makes your choice simpler.
It's a common mistake, due to Guzzi themselves labelling them differently at different times, they are all 4-Valves-per-Head
Engines, hence the 'Quattro Valvole
' label on the Rocker Covers.
The 8V Engine will 'cruise' all you like, and return great fuel mileage at 'sedate' speeds (60-70mph).
I can get almost
200 miles from my Small Tank Stelvio on a long steady run.
192 miles is my personal best so far, and I'm determined to crack the 'Double-Ton' one day.
The problem is, the Engine is so intoxicating to rev beyond half-way on the Tacho, it's difficult to restrain the urge for a full Tank's worth of riding!
The 2011 onwards Stelvios have the 32 litre Tank (17 litre on the 2008-2010 models).
It adds a bit of weight up high, but only when full, and of course you don't have
to fill it up every time.
They are amazing Bikes, they just do it all, no matter what you throw at (or on) them.
As I understand it, Guzzi recommend against
fitting a 'fixed' Sidecar, due to the extra strain they put on the Swing-arm and/or Final Drive Unit.
Having said that, there are plenty around with them fitted, as a quick Google image search will show, and I've yet to hear/read of any having issues.
An option might be a Kalich style Leaner Sidecar.
Almost the same carrying capacity as a fixed Chair, but on/off in 15 mins, and still rides like a Solo.