ditto ... I'd probably go for three pieces, somewhat equal width, if I wanted a one fabric back.
That being said, ....... have you considered using something from the front to add in that extra 8"? Might be a strip of the fabric? or perhaps some extra blocks from the front? (I know that some quilts, one wants the plain back and this would not even be an option.)
Here's what I did for a backing when I had much the same predicament as you ..........
[ATTACH=CONFIG]435798[/ATTACH]
And here's what the front looked like ....
[ATTACH=CONFIG]435799[/ATTACH]