Best way to get the ratio is pull the cover. If it's an original it probably could use a lube change anyway. On the edge of the ring gear there should be some numbers stamped. 42/19, 42-19 or the like. Just divide and that'll give you the ratio. Axle codes are fine but after that long you don't want to assume a past owner didn't swap gears. Easiest way is Q's method. It'll get you pretty close.
