Swapping cranks, but the new crank is wider, how do I compensate for this?
You're right, you want to space it between the shell and cup.  You can put spacers on either side to get the chainline right.  Most cranksets come with spacers (2.5 mm is probably the most useful size for you) but if yours didn't any LBS should have a few to sell you.  Something like this: http://wheelsmfg.com/bottom-bracket-spacer.html (I don't know if these work with hollowtech BB's or not)

You'll probably want to get an assortment of sizes if you can, and use as few as possible to get things lined up (I assume this would help maximize BB stiffness, but really I just hate having excessive spacers - makes it more likely I'll lose one when disassembling/reassembling).