I don't think that 1/2 a size needle would make all that much difference. It is an option, though, to go down in needle size and stick to the pattern.
You're right that the sides won't match, but if you don't care, I'm sure nobody else would even notice.
Are you sure you can live with the back being 10 inches too wide, though?
It would be as much of a shame to reknit as it would to finish the front and find out you're unhappy with it. Sometimes frogging and fixing it makes me feel better in the end.
That's your call, though.:hug: