First, if you search here, the Fat Tire recipe has been covered probably as much as any I have seen on this site in my short time here. That being said, while the recipe you list is very close to others I have seen, I don't think it will be dark enough. most recipes call for small quantities of chocolate malt to get that rich color. However, I had some very interesting discussions with one of the New Belgian brewers over the winter who said they do not use chocolate (due to its astringent flavor characteristics) but rather a darker crystal malt to achieve that color and maintain malty sweetness. So, I'd substitute the crystal 40 (and possible the 20) for 80 or 100--best way would be to use pro mash or similar program to try and achieve the same SRM. I made mine with all Willamette and was happy with the hops but the brewer suggested a second hop of an "earthy" variety (I beleive Fuggles was suggested).
Hope this helps.