If its mm your looking at I can suggest getting to 420mm using Nikon lens instead
Take a strong look at the Nikon 70-300mm VR lens instead. Hold on a minute before you say "but its only 300mm"
What you wont find this tip anywhere but works is adding a teleconverter (I used a Sigma 1.4 TC) to it. this gives a range of 70 -420mm which is pretty close to what your after. This also means your not going as high as f6 with the sigma and greater range of mm as well. That mm length is for an FF camera so on a cropped sensor like the D90 you should be getting very close to the 500 mm mark anyway. Havn't worked out the maths for that
Why do I say this? well the "throat" of the inner workings are deep enough to not foul the TC which is about the only lens that allows it. Yes top of the range lenses allow TC'S but we are not talking about that here.
The reason why you won't find this recommendation anywhere is because of the f stop used means it will either work with auto focus or manual focus. But this in my opinion is a small price to pay for the same result more or less
I have successfully used this combo many times when I wanted that bit extra length. Of course you could go up to a 2x TC but the light loss onto the sensor is too great for all but the brightest sunlight which is why i got the 1.4 TC.
Cost Nikon lens 70-300mm VR lens = £399
Sigma 1.4x Tele conv =£200
Little bit cheaper than the lens in your link
Below this combo ,note the lens is at its shortest 70mm
Photo using this combo taken from other side of large wild life enclosure (hand held). without this combo this picture couldn't be achieve as the wolf was too far away

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