Agree 60 amps with good idle output is OK for a stock muscle car engine... including with AC.

My 400 (based) engine is not stock. Big EFI fuel pump and electronics draws some power. A 75 amp upgraded squareback barely kept up running down the highway at night and AC running. At idle a big drain and voltage drop.

My current "fix" is a later 70's "big Chrysler" alternator. 100+ amps and lots of idle output. Works fine... just heavy and unique brackets which I already had.

I may try one of these... supposed to bolt in place of a square back.

http://www.qualitypowerauto.com/item_557/Chrysler-Mount-Mega-Amp-Alternator.htm