There were two types of differentials in '68. One was the "O" axle with gear ratios of 2.56, 2.78, 3.08, 3.23, 3.42, and 3.91 in either an "open" or "anti-spin" version. There was also an "O" axle 4.33 gear in just the "anti-spin" version (I believe the 4.33 gears were dealer-installed). The other type of differential was referred to as the "C" axle and was used in Canadian built vehicles in gear ratios 2.56, 2.73, 3.07 and 3.31, in either "open" or "anti-spin".
There should be a two-letter or a letter-digit code on your axle housing that will tell you what gear ratio you have. Most of the time I've found it stamped upside-down on the passenger side forward.
Check the 7th figure in your VIN. If it is the number "1", your car is Canadian-built. If there is a letter in that position, your car was built in the USA and that letter will tell you which assembly plant built your car.
As "wmachine" said, a great place to go is www.wildaboutcars.com
. There is a host of info at that site that you can view and download for free. Signing up is also free. I highly recommend it!
'68 4-4-2 convertible
'69 4-4-2 convertible
'70 GTO hardtop
'71 Corvette coupe