My 71 Chevelle has manual drum brakes. How can I convert to power assist?

Get a booster and put it on. You will need some new lines that go from the master cylinder to the proportioning valve also. You can actually get a new booster with a new master cylinder on it from pretty much any auto parts store. I paid like $130 for one.

MP brakes has a gm booster and master cylinder for $125. Its a really nice unit and the bracket comes out from the fire wall more so its easier to bolt on or get off if necessary. I took my original brake lines to a hose place that extended them and added the neccesary residual valves. MP brakes sells the valves also.