Pinion snubbers used to be quite common in some drag racing classes. Might even be some pictures in the appropriate Mopar Performance catalog?
Remember, too, that a pinion snubber can make the rear suspension "bottom out" sooner. Rather than use the center snubber, also extend the outboard snubbers so that they all contact the rear axle items at the same time, not just the center part.
"Length"? It was usually more concerned with "height" and the distance between the rubber part and the "crash pad" on the vehicle's floor pan.
As rear suspension "science" evolved, it was more abouit "shocking" the rear tires with sudden applications of torque at the starting line. With an automatic transmission car, trans brakes and pinion brakes were usually applied and then suddenly released. 4-Link rear suspensions might have worked better than leaf springs for these things, possibly? As the links would not deform as the leaf springs would end up in an S-shape when all of the torque was released.
On leaf spring cars, as things evolved, it became popular to take the clamps off of the rear of the leaf springs, to allow the rear section of the springs to separate under torque, at the starting line. Quite a sight to see the rear leaves have more than 1" between them at launch! Don't know that I'd do that on a car that usually sees "street time"?
FWIW,
CBODY67