Mushy stock rubber transmission mounts can't take the torque and vibration of high performance street and race use. Energy Suspension polyurethane transmission mounts can. Polyurethane lasts longer than rubber, can handle more horsepower, and is soft enough to absorb vibrations.
This mount works for 1958 to 1982 GM Transmissions:
All Borg-Warner, Muncie, Saginaw, Chrysler 4-Speed
7/16"-14 thread size
Bolt hole centers for crossmember is 1.5".