It should be fine. Note that I have 8.5" rims for my fronts at +35 offset and added a 1" (25mm) Adapter in order to get them where they're at right now and they still don't stick out of the wheel well. As you noted, my front rims are not as concaved as the rears and are probably closer in depth to yours. If I wanted to get my fronts to where they're at now without the adapter, I would have to go +10 offset (25mm difference), or get a 9" wheel going +20 offset (about 1/4" or about 10mm difference). Or go 10" wheel with a +35 offset (about 1/2" difference)... and with a 10" wheel for the front I am probably risking some rubbing.
Go for the +20's...
Edit: Corrected the math above.