The turbo 350 and turbo 400 are GM transmissions that were used from 1969 through 1979 for the TH-350 and 1964 through 1990 for the TH-400. The TH-350 was a low-torque transmission that was used in heavy cars. With modification, the TH-350 could be used as a high-torque transmission in a light car. The TH-400 was a high-torque transmission that was used in heavy cars — it was a much stronger transmission than the TH-350.
Locate the vacuum modulator. On the TH-350, it is located on the right front of the transmission. On the TH-400, it was located on the right rear of the transmission. Also, the TH-350 had a kick down cable, while the TH-400 did not. Both were designed to be used in Chevrolet V-8 cars and light trucks.