I think I get it now :P
I'm not totally sure why you would need a duct larger than 80mm though, the 120mm will be restricted by the radiator (different kidna will cause different amounts of restriction). I doubt it would be further restricted by a duct that's only 80mm, obvioulsy you would still need an adaptor, but you might not - I'll explain in a pic.
If you have the 80mm fan, you dont need to connect the duct to the rad - because the 80mm will push the air up towards the rad and the 120 will just suck it in if that cool air is there.
If you do connect the duct, you really dont need the fan, the 120 will suck the air all the way from the side of your case. Also depending on the fan speed and air restriction an 80mm duct might be adequit. If it were me, I would duct it but leave only the pulling 120mm fan. I also think that the 80mm pushing, without connection could work well, it would pump cool air up there, the 120 would suck most of it in (note: if it sucked more than all of it in, then you deinately do NOT need that 80mm at all not that you'd be able to prove any of this :P) if it didn't suck it all in, you're dumping cool air into your case, which never hurts.