You could use a duct collar to connect the two, but some of that aluminium foil tape which sticks very well and is strong would work provided the two didn't move around.
As far as noise goes, acoustic ducting can reduce noise, but 1, 6" acoustic is 8" thick, 2, such a short length wouldn't make much difference.
If you want to lower the noise connect it to a variable transformer speed controller and turn it down. 6" is huge for a small cab, I run a 5" in my 4x4x5 veg tent and turn it down to 2 (of 5) in winter and run it on 3 in summer. That makes a huge difference to sound levels. Also consider that a 6" on full power requires a fair amount of passive intake holes to allow the fan to work effectively without choking it and creating more noise.