The tonality of Polaroid 669 isn't too far off from Portra 160 NC, but neg film has more latitude in general. You'll need to run some tests and do some calibration in any case to be able to rely on Polaroid or digital for that matter as a way of proofing for exposure and contrast. The way to do this might be to bracket a series of exposures on the neg film of your choice, taking notes, and then do the same with Polaroid, develop the film and run a contact sheet (not machine proofs, because they might adjust the print exposure for each frame), and compare the Polaroids to the contacts, and find the best match. When you've figured out the exposure difference between the neg film and the Polaroid, you might use a ND filter to match them, so you can shoot the proof and the real shot at the same aperture and shutter speed.
One thing to note is that Polaroid doesn't have great reciprocity characteristics, so it's a handy proofing medium for daylight and strobes, but it's not as easy to use for long exposures (though architectural photographers do use it for complicated interior shots involving mixed lighting and multiple exposures with different filtrations).