I've used the Drops Alpaca yarn before some years ago when I made hats and scarves for the niecelets one Christmas, and it's a lovely fine yarn that relaxes quite dramatically when washed. Hence I'm trusting that the neck is going to be a bit looser than it is at the moment when it's finally done and blocked. I did make at least three swatches for this (washed and blocked too), so I'm fairly confident it will work out.
Of course the great thing about doing a top-down circular knit is that you get to try it on as you go along. The bad thing is that by the time I get to the sleeves it's not going to be a very portable project.
The pattern is Hannah Fettig's Lightweight Pullover which you can buy as a single pattern on Ravelry or it's available in this book if you're not a Raveler (if not, why not?!)