Update!

Well, I got my ES51 68x121 BB today.
Turns out it isn't symmetrical...
The drive side is 4 mm longer than the left side.
Since the entire axle is 8 mm longer than my 113 mm one, that means the right side is actually 6 mm farther out than my current one.

Now, the chainline was measured to be 42.5-43 mm. Adding 6 mm will bring that to 48.5-49 mm. Right in the middle of the 47.5-50 mm range suggested on Sheldon Brown's site.

Now, if only I'd known that pulling a crank was so damn hard when I last ordered parts... I'm sitting here without a crank puller, and no bike shops near by.
If I'm lucky, a friend om my brother's has one, but otherwise, it's a nice 7-14-day wait for me until I can get a crank puller...