If that 78 amp unit is the old three wire version then what you have is a good paper weight. Even if it is in top charging condition, it does not put out enough at idle to keep up with all the demands on it. You do not mention what you have on the dash to monitor it, idiot light or gauge. A gauge, at idle, with everything, lights wipers, A/C, et all would likely show a reading below 12 Volts, meaning more power is being consumed than the alternator is putting back. Having gone through multiple 78 amp GM alternators over the decades, as fast as I could score one from the yard I went to 100 amp or heavier. The other option is to go aftermarket and get rid of the three wire harness. The newer alternators are internally exciting, like the 12si's, and will immediately throw a charge instead of you having to goose the throttle at start up to pop the rpm's over 2 grand to get the alternator to "wake up". At last count I think I have at least 6 of the old school low amp A/C Delco's gathering dust in various places. I don't get any demand for them, even as cores and most of them still generate power, just not enough and consistently to satisfy modern demand. On the other hand, if you know someone building a bare bones hot rod then a 78 will work just fine for them.
Nick