Cannondales have stiffer chainstays than most steel frames but they will still flex easily enough to accommodate the wider hub. I did the calculation once, as someone had posted some fear-mongering link off the internet suggesting that bad things would happen if the dropouts were not realigned. IIRC, assuming the dropouts were perfectly straight to start (unlikely), they'd sit at ~0.5 degrees after being stretched. If you can eyeball that, you are better than me! As noted already, don't even think twice about it. It will be fine.
I can't let a question go by about going threadless on a vintage frame without commenting, or at least inserting a pic. Below is an SR500 I built up on a budget using some Mavic wheels (early 2000s but Mavic got lucky and made them with a freehub that accepts modern 11 speed cassettes) and Shimano 105 5800. The headset is something I designed allowing threadless stems to be cleanly fitted to threaded forks. These old fat-tubed Cannondales work well for this type of conversion, in my opinion. I left it uncut for this pic but the stem can be 'slammed' down on top of the upper bearing or fitted with up to 50mm of spacers as shown.