The yarn you want to use is 6 stitches to 4" on 10.5 needles which is considerably different than 12 stitches to 4" on the project gauge. I would do a swatch and see how close you come to gauge for the project and see if you can go down a size or 2 on needle size without changing the texture and drape of the project too much for comfort. Otherwise I would look for a different project or different yarn.
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