All you'll need to know are transmission gear ratios, differential gear ratio, and tire diameter. Always measure tire diameter with the tires at proper inflation and mounted on the car, because ...

If you’re paying to do a gear change, you may want to take advantage of the fact that it will be little or no cost in labor to add a locking or limited slip differential at this point.

That style is still available from Iskenderian (PN RATIOCOMP). In a more modern vein, Summit Racing has an online gear-ratio calculator .