How do you change the vacuum modulator on a 1978 Chevrolet C20 pu 350 AT Turbo400?

The vacuum modulator is on the passenger side under the cooler lines on a turbo 400. It's held in with one bolt that goes through a clamp that holds the modulator in. To change it put a drain pan under the modulator, pull the vacuum line off it, remove the bolt and clamp, pull the modulator out. Make sure the o-ring seal comes out with it, be sure there's a new o-ring on the new modulator and put it in, replace the clamp and bolt that holds it in and hook up the vacuum line (if it's an adjustable replacement modulator you might need to use a little bit longer rubber line if it has a steel line running down with a short rubber line at the end since some of the old turbo 400's originally had a big canister modulator and the new one will probably be smaller).

Then start the engine, check for leaks and refill the transmission as needed.

Same procedure on a turbo 350 but the modulator is in the rear on the passenger side pointing towards the back of the car/truck on a turbo 350.