IMO I would keep your 67 setup - I have been extremely happy with both my 67 kit and a complimenting 645Nii body. I have no complaints whatsover using 67 glass on the 645nii - on the contrary the Fotodiox adapter works perfectly and you retain auto-aperture. I still do not have any dedicated 645 glass as I've just not felt the need.
Pentax 645Nii | SMC-67 45/4.0 | Velvia 100F | V750 scan
Pentax 645Nii | SMC-67 75/4.5 | Velvia 100F | V750 scan
Pentax 645Nii | SMC-67 75/4.5 | HP5 Plus push+2 | V750 scan
Pentax 645Nii | SMC-67 165/2.8 | Fuji Pro400H | V750 scan
For what it's worth.