[66 Boid]

Greetings,
I am having my 1966 T-Bird's rear-end rebuilt. It was making some serious whinny noises.
Along with changing the bearings and such, my "Bird Mechanic" suggested to change my gears from the original "300's" to "312's".
As I am not really a mechanic myself, could someone explian the difference between the two? My T-Bird is a very nice driver, but is not a show car, so I am not really worried about keeping that portion original.
(The rest of the car is,though)
Thanks in advance for your time and replies.
Tom