If there is no way to identify the rearend ratio you could jack up the rear end, put transmission in neutral, block one wheel so it won't rotate, mark the driveshaft and count revolutions while helper rotates the other wheel one full revolution then multiply driveshaft rotations x2, thats assuming you have open rearend. To tell if you have open rear or trac loc jack up rear with both wheels off ground trans in neutral, rotate one rear wheel if the other spins in opposite direction you have open rear if they spin in the same direction you have trac loc. if you have trac loc you don't have to do the 2x multiplication but both wheels need to be off the ground and spin equally in the same direction. To figure the ratio a little more accurately you could rotate the rear wheel 10 full rotations, multiply driveshft revs x2 then divide by 10.
Example for open rear: 10 rotations of rear wheel=18.65 rotations of driveshaft. 18.65 x2 = 37.3 /10 = 3.73 rear ratio.
On the bias side I agree with what others have posted, they may have nostalgic look but at a great sacrafice to safety and handling, especially on these cars that are not great in that department anyway. The story in one of your other posts of having to hit the brakes hard to dodge some jerk that pulled in front of you comes to mind. . . . . ok, ok so you're not going to pay attention. . . . I like blacks.