If it's still there you may find a tag---or an applied little decal with the ratio. Easy way to figure it out. Jack up the car (properly supported of course). You can determine if your car has LSD (not standard) by rotating one wheel--if the other wheel spins in the same direction you have LSD - if it spins in the opposite direction you have an open dif.
Mark the driveshaft with chalk and rotate one wheel through one revolution--counting the revs of the driveshaft -- multiply the driveshaft revs by 2 and that's your ratio. A 4.30 will spin more than 2 times--a 3.91 less than 2.
You can add an LSD -- I pulled one from a '76 265 and installed it in a '73 140 diff. using the original 140 gears. -- Dave