I agree with Sue.
This garment should not fit sloppy.
A little ease can be accomplished with a slightly larger needle.
Make a couple of test gauge swatches in the berry stitch using different needles.
Then do the math to see how much bigger chest you will have by using needle "A" or "B" or "C".
You shouldn't have to go up in gauge or needle size too much to get a couple of more inches around when knitting the size 40". Remember, both the FRONT and BACK will be a bit wider. A slightly larger 'gauge' will spread out over both pieces. Is the BACK plain st st? If so, then make a st st gauge swatch, too, for testing purposes. It's a shame the pattern didn't have a 43" size. The bust area is supposed to fit snug, as per pattern photo.
This garment is adorable, as are you. You are a perfect match!
It will be worth the effort!