1. Transmission Selection:
* Identify the correct TH transmission: Not all TH transmissions are created equal. The optimal choice depends on your horsepower and torque goals. A TH350 is common for many 350 small-block applications, offering a good balance of performance and durability. A TH400 is more robust and handles higher torque but is larger and heavier.
2. Required Parts:
* Transmission: The chosen TH transmission (rebuilt or new is preferable).
* Torque Converter: A torque converter matched to your engine and transmission. The stall speed will influence performance characteristics.
* Transmission Mount: A new transmission mount designed for the TH transmission and your Impala's frame.
* Driveshaft: The existing driveshaft will almost certainly be the wrong length for the new transmission and likely needs to be shortened or replaced.
* Crossmember: You'll likely need a new or modified crossmember to properly support the TH transmission.
* Shift Linkage/Cable: A shift linkage or cable specific to your chosen transmission. This will require adapting to your Impala's interior. Floor shifters are common, but column shifters are possible, although more challenging.
* Kickdown Cable/Switch (if applicable): Depending on the year and configuration of the TH transmission, a kickdown cable might be required for automatic downshifts under acceleration.
* Fluid Cooler Lines: Lines to connect the transmission to the radiator for cooling.
* Electrical Connections: Wiring for the transmission's electrical components (e.g., neutral safety switch, backup lights).
3. Installation Process:
* Removal of the 4-speed: This includes disconnecting the clutch linkage, driveshaft, and bellhousing.
* Engine to Transmission Adapter (possibly): While likely not required with a small-block 350, double check compatibility of your engine's bellhousing with the chosen TH. Sometimes an adapter is needed.
* Transmission Installation: Carefully install the new transmission, ensuring proper alignment and mounting.
* Torque Converter Installation: This requires special tools and careful procedure to avoid damage.
* Driveshaft Installation: The driveshaft needs to be shortened (by a driveshaft shop) or replaced with the correct length.
* Crossmember Installation (or Modification): This provides proper support for the transmission.
* Shift Linkage/Cable Installation: This will require precise adjustment for smooth shifting.
* Fluid Cooler Line Installation: Connect the transmission cooler lines to the radiator.
* Electrical Connections: Connect the necessary electrical components.
4. Considerations:
* Professional Help: This conversion is best done by a skilled mechanic experienced with classic car transmissions. Improper installation can lead to serious damage.
* Cost: Expect significant costs for parts, labor, and potentially specialized tools.
* Performance Tuning: After installation, you might need to adjust the carburetor or fuel injection system for optimal performance with the automatic transmission.
In short: Converting your 1967 Impala to an automatic transmission is a substantial project. It's not something to undertake lightly. Thorough research, planning, and the right tools and expertise are crucial for a successful outcome. Consider consulting with a professional transmission shop for estimates and advice before starting this project.
