You can 'make' a slightly shorter MTX gearbox. Lee at Devil Developements did this when he fitted a diesel Focus MTX box to his Mk6 Fiesta. He made all the mounts etc... but when he got hold of a Focus RS box he found it was slightly longer and fouled on the inner wing.

In the end the end he swapped over the ATB into the diesel box as he plans on cracking 200mph with it (currently building a Nikasil linered 600bhp Zetec turbo!).

I was under the impression they were all the same dimensions but apprentely not!