According to the contract documents as scoured by MustangsDaily (and confirmed by us [PDF link]), part of the deal involves an upgrade at the Romeo Engine Plant to manufacture a new 5.8-liter supercharged V-8 engine that will replace the current 5.4-liter supercharged unit. The current GT500 uses the 5.4-liter aluminum-block engine.
The obvious implication is that the next GT500, which is already expected to grow in power, potentially up to 600 horsepower, will use the 5.8-liter supercharged V-8. A 400-cc increase in displacement could be enough to get the 50-plus-horsepower gains expected.
Previous reports have also hinted at precisely this upgrade, but the finding in the UAW contract documents (full contract in PDF form here) firms up the case for the larger V-8.