Oh, brother -- confusion and misinformation.
The SX-70 will overexpose by two stops with the dial centered and 600 film loaded (after you use the "shoehorn" trick or carve off the bumps on the pack in order to load the new film into the old camera). Putting a neutral density over the meter will make it overexpose even MORE. Putting a two stop filter (ND .6) over the *lens* will correct exposure for both daylight and flash, but at the cost of severely darkening the viewfinder on SLR models.
The simplest way to correct the exposure for 600 film in an SX-70 without darkening the viewfinder is to remove the front panel, REMOVE the filter (1 stop, i.e. ND .3) that's already present over the meter cell, and then (after replacing the panel) turn the dial all the way to *darken*. However, this leaves no leeway for reducing exposure in situations where you want a darker or more saturated print. This can be resolved by then putting ND .3 over the *lens* and using the camera with the dial centered, though that will again darken the viewfinder and make it difficult to focus in poor light.
There's also a possible modification of the metering system by replacing a component (a capacitor, IIRC) with a smaller value item, which (if the correct component value can be found) would allow using the camera with dial centered and no filters. All of these metering modifications, however, will destroy the ability to use the camera with flash, as the flash exposure is taken from focus distance and will be two stops too great (or one stop, if you have an ND .3 on the lens). Another filter, or a diffuser with filter, on the flash could correct this.
One method I've talked about a few times but don't know (yet) of anyone trying is to put a piece of ND .6 sheet material over the opening in the film pack when loading the film; if taped in place so it doesn't crease or pull out of place, this should allow using the 600 film in an unmodified camera, including with flash, without darkening the viewfinder -- with the proviso that you'd probably have to replace the filter sheet each time you load a fresh film pack (however, Lee theatrical filters come in a 24x24 inch sheet that will yield up to 56 3x3 inch filters for $7, and they do offer ND .6 in this form).
None of this will save the manipulability of the Time-Zero film, of course, but it could keep some of these old classic Polaroids working (hard to believe these techno-marvels were introduced more than 30 years ago).