I don't do a lot of portraiture, but I bought a 67 105mm f/2.4 specifically for that purpose some time ago, after seeing Chris Willson's early work with it on his 645D. I have the newer version of the lens. KEH.com has some right now; they call the newer one "PENTAX 105MM F/2.4 SMC LATE LENS FOR PENTAX 6X7 SERIES". The older one is the Takumar, I haven't tried it.
Here's something I made with the 105mm on the 645Z. This was shot at f/8; available light, ISO 400, handheld. I wanted a little more depth of field for the subject so I didn't shoot a wider aperture. I do like the look of this lens. Nailing the focus right on is challenging at the wider apertures, but like most things, with practice you should get the hang of it.
The new 645 90mm f/2.8 is probably better all around. But for the price difference there's hardly any downside to trying the old 67 option.