adding front panel to tunic?

I just bought a lovely linen tunic from the charity shop, but it is actually a bit too tight across the bust.  There is a small V-shaped slit on the front, and I'm thinking if I can cut that lower down, and add some sort of panel to the front, that might take care of the problem.  Does anybody have any advice or tutorials about this sort of modification?

Thanks!
Rachel

Edited to Add: Maybe something like this? 
http://www.associatedcontent.com/article/2809522/triangular_insert_for_toosmall_dresses.html?cat=46

As clever as that solution sounds, I honestly think it would distort the original style of the garment.

Alternatively, you might be able to borrow from the side seam allowances in the bust area.  Just unpick them and resew them at a narrower clip.  Even if you get just 1/4" out of it from the front and back, that'll give you an extra inch of movement without changing the look of the original garment.  I often do something very similar to this when I'm making up a garment to avoid full bust alterations, which end up being really awful on me because they add width in the waist and hips which I don't need.

Thanks, that is really useful advice.  Will give this a go and let you know how it works.  At the very least, it's less likely to ruin the garment than what I was thinking!