This bit i dont understand, cause the MTX will bolt up to a blacktop zetec E as used in later mondeo and the focus, and i've also got an IB5 bolted to the same engine, so surely the MTX and IB5 share the same bolt pattern.
At least on the block the pattern is the same i'm sure, the lower half bolts in the sump will be slightly different due to needing different shape sumps, bit i have it on good knowledge that the 5 bolts holding gearbox to engine block are enough to cause no problems, only 3 from my alloy sump wont line up.
So in theory, any IB5 or BC box should fit, driveshafts permitting, if you're willing to leave a couple bolts out when they dont line up, since the block is a hefty chunk that the majority of bolts will be fixing to.
Yes! That's why it is called IB5+.
MTX fits to the engine, but only the ones that were used for Duratec-HE engines in Focus, Mondeo, C-Max... And you'll need new driveshafts and probably many other things!
Don't mix it up with other engines Ford nowadays calls Duratec! Only the chain driven Duratec-HE (I4, 16V, front inlet, 1.8, 2.0 and 2.3) are the same engines like your ST has.
But yes, it is possible to put the internals of another IB5 into your ST box. And I believe just swapping the engine part of the bellhousing from IB5+ to IB5 might work. But you'll also have to take both boxes apart for this.