Fitting a Fiamma rear box to T5 Trident

Hi,
my first posting, just joined
 can anyone help me, is it possible to fit a Fiamma ultrabox to the rear of my T5 Trident or will the box hit the rear of the Hi-Top when the tailgate is opened?
When I recently aquired the vehicle it came fitted with a Fiamma Carrybike rack (not that I have a bike lol).
There is a recess molded into the rear of the Hi-top roof,this accomodates the top rail of the bike rack when the tailgate is fully open.
The ultra box 320 or 360 are 700mm tall which looks a bit close and I think it may hit the roof. I haven't got one yet so can't be sure if it'll fit.
Maybe if the bike gutters are removed does that help to lower the height of the box or do I have to leave them on to rest the box on?
All advice gratefully received.