Phildar Cardigan, or, somebody block me!

So, I have a dilemma, and I need the guidance of the ever helpful knitblogosphere.

I have all the pieces of my Phildar Asymmetrical Cardigan finished! Hoorah! Finally!

Ok, so. Here is what it's supposed to look like:



And here's what all the pieces look like: (sorry for the craptastic photos. It gets dark so early!)


Two freakishly long sleeves, two fronts, two buttonbands and a back. (Yes, I still have to bind off one of the buttonbands. Sue me.)

Here's the dilemma. The whole sweater is 3x3 ribbing. Which seriously diminishes the size of each piece. Allow me to demonstrate.

The back, in its natural state:


And the back, quickly pinned out to the required dimensions (with camera case for scale).

Dude, that's a lot of stretching.

So, what would you, dear readers, recommend as a method of blocking this thing?
I want it to be close to the required size, and need to have a good edge for seaming, but I don't want it to be blocked so harshly that it doesn't have any give left. I'm leaning toward steaming it, but would like your feedback.

Any thoughts, input, advice, comments or anectdotes welcome.
Thanks!