I would think that finding a 6 cyl Rally Pac would be like looking for the Ol' needle in a haystack. There out there I'm sure, but I have not come across one in years. Someone like the tachman or clock doc would probably be able to convert an 8 cyl Rally Pac tach to 6 cyl operation for you.
I believe converting a Rally Pac to 6 cyl. use, only requires a resistor across the two tach leads. I am not sure what value resistor, but I am sure you could get the parts at Radio Shack for under 5 bucks. I think I read how to do this conversion years ago in Mustang Monthly.