I don't know set you are on using the H55 but DieselPumpUK also sells an adapter kit to bolt the OM606 to an H15X transmission utilizing a diesel 80 Series bellhousing. If you can find an HZ to H55 bell housing you could probably use the same adapter kit. It might be easier to source the parts...
Fantastic writeup, I've probably read through it 2 or 3 times over the last couple months! I'm about to start gathering parts for my own OM606 swap into my 80 series. DieselPumpUK makes an adapter/clutch kit so we can now use an H15X or R15X gearbox. I love the idea of using a mechanical pump...
I'm definitely going to be following this build! Planning a 606 swap with the doomsday kit myself. Did you looks at any of the turbo offerings from DieselpumkUK? They've got a Holset HX30 kit that claims spool as low as 1900RPM and a hybrid kit that's even sooner than that.