Might poke around at www.novaresource.org & see what you find. I'd guess 2.73 or 3.08, but it may not be original.
Or jack up one rear wheel (trans in neutral, front wheels chocked) & count how many times the driveshaft turns when you rotate the wheel TWO full turns. That's your ratio. Wheel won't turn? Release the parking brake, or you may have a posi rear. Jack up both rear wheels. If you turn one & the other turns the same way, you have posi. If so, turn the rear wheel only ONE full turn & count driveshaft revs to get your ratio.