He can use the power booster as a manual. He will have to watch his pedal ratio. The manual brake cars have the pushrod mounted higher up on the pedal. This gives more pedal power, at the expense of a greater distance of pedal travel. I'm not sure on a 67 Imperial, but some other mopars have a cantilever that lowers the pushrod from the manual position to the power position. If so, this can be removed and the pushrod mounted direct to correct the ratio.