Should be a Ford 9" rear which is practically indestructible. I'm guessing it is an open rear (non posi) so what user151 says is the correct way to check the gear ratio. The best way is to jack up the passenger's rear tire (right rear) since that is the drive wheel, and make sure you have some sort of chalks holding the front tires so the car won't roll off the jack. With the trans in neutral rotate the tire twice and count the driveshaft rotations. That will give you the ratio. If he can't turn the wheel at all or if it is very difficult to turn he might have a posi rear, in which case you'll need to jack up both rear wheels and turn either wheel one revolution to get the gear ratio. I'm guessing since the tag is still there it has the stock 3.10 gear set which as has been mentioned is a little tall for an overdrive trans, however he can have the gear set changed to something a little more OD friendly pretty easily.
The most important things you learn are those things you learn AFTER you know it all. -- Chip Foose