I designed an app on the phone and am moving it to tablet. Testing on 3.2, a Samsung something-or-other. The legacy menu doesn’t fill the screen using existing resources…

So, notwithstanding this, how can I make my menu fill up the horizontal space? (Besides playing tricks with wide images, if that would even work.) Again, I’m not interested (yet) in doing the action bar overflow thing.
This is not supported by the SDK. The layout of a menu is determined by the OS, occasionally modified by the device manufacturer. You, as an SDK app developer, do not have control over sizing, number of rows/columns, etc. In effect, the “menu” is an abstraction — you simply indicate what items should be in there, and the visual representation of that is up to Android. That’s why, for API Level 11+, there are zero code changes required to use an action bar, other than to have
android:targetSdkVersionset to 11 or higher.