Yes I meant power take off for the rear.LOL. I read it on my phone and in a hurry and read nineties rear diff when you actually said noughties rear diff. Sorry.
I think the 164 Q4 gearbox was developed in the 90s.
Not that it would matter. I don't think it makes any difference as it's mechanical. Look at the T56 gearbox, which was developed around the same time as the 164 Q4 and was used in sports cars such as the Corvette and Aston Martins all the way up to the late 2000s. A manual gearbox is a manual gearbox.
As far as I understand, the 164 Q4 transferbox is off to the side of the gearbox. If by transfer case you mean the power take off port for the prop shaft.
Instead of a mid diff, it uses a viscous coupling system, which is electronically controlled. So it's better to replace that with something mechanical to control the split anyway, like what the 156 Q4 has. And the viscous coupling is at the rear with the torsen rear diff.
So as far as where to put the a mid diff, what about in the middle? I think a better question is, which mid diff to use.
The 164 Q4 came out in 93 so it was probably being developed in the 80s and 90s although as its a getrag box, it had been around before. And as it was used in other cars there may be more modern version of it, if you can get a 164 transfer box to use with it.
But will the 164 transfer box line up with the 156 Q4 diff ? I guess it doesnt matter as youre gonna need a different prop or at least mod the original anway - youve got to divide it add a centre diff, which hopefully wont need any of the body cutting away to make it fit in the centre tunnel.
Youd deffo have to replace it no one uses viscous couplings anymore plus it electronically controlled and youd have to add those 90s electronics to the car.