Both the 4.5 and the 5.6 are VK motors and if the M already accepts the 4.5 then the 5.6 shouldn't require too much fab work if any. The 5.6 is a bored and stroked version of the 4.5 so the exterior dimensions are mostly unchanged.
Forget the 5.6 though. We all know what motor will end up under the hood of the next gen V8 M. The 390hp 5.0L VK50VE from the '09 FX.
