/// I am suggesting that the rear end gear ratio must be the same for the same performance. The ratio for a manual transmission is different than for an automatic transmission. Limited slip is another matter entirely. Volvos usually have a plate on the back of the driver side axle tube that states the ratio. If not, you can count the turns the wheel makes to make the drive shaft turn over once. Block one wheel. I don't know what they would be for the 1800. Try the archive.

On these older cars it's best to swap the whole used rear axle unless you have the entire rear end rebuilt and the gears installed professionally. Old gears have an established wear pattern. New parts or different used parts won't match that pattern.