Can't imagine why that would work. You will get the same plus rate at the grey wire as you would at the neg of the coil and will likely not get enough voltage on the grey wire to trigger the tach. I have never had an issue with tachs working properly on the neg of the coil regardless of the mopar box.
That being said I played with a factory tach on my 68 Barracuda and gave up because the movement was so slow it couldn't keep up. I purchased a reproduction tach with a modern movement to go in the hole in the dash.
If you want to try and get the factory tach working these guys have a replacement circuit board that has two pots that you adjust to calibrate it.
