The 240mm aspect is only for mounting - the Rad can always be marginally bigger. In some cases, there's still sufficient clearance for the last 120mm fan, in others, there's not. Sometimes you can move the Rad slightly higher/lower, if the fan mounting points are rails opposed to fixed..... not 100% sure on the Spec06 though.
The Spec-O6 supports only 120mm or 140mm fans up front. While an 80mm might 'fit', it won't align with mounting points.
Even if it could fit, the cooling ability of an 80mm fan, plus the added noise it would generate is probably not worth installing anyway.