Ventura luggage for Griso?

I know it’s not the best bike for touring but its going to have to do for the Club Festival rally. I live in North Yorkshire so guess its around 200+ miles trip.
I normally bungee an Orlieb waterproof rollbag on the back of the seat for local rallies etc but it does limit the kit I can take.
Does anyone have experience of the Ventura luggage system on a Griso with regard to fitting?
I did have it a few years ago on my CB1300 and it worked well for a France tour of two weeks.

I may have to carry extra fuel as I don’t expect the Griso to return more than 100 miles to a tank and the reserve it not that much. I’d hate to run out of fuel on the A1/M42/M5 between service areas.

Again, if anyone knows better, let me know.