What rear is it? 8.2", 8.5", 7.5", 12-bolt? We need more info. Also, what's the casting date?
JA = 2.73 open in a 1973-74 Nova or 2.56 open in a 1975 Nova
G = Detroit Gear and Axle assembly plant
178 = assembly date, 178th day of the year (June 27th or June 26th of a leap year)
4 = ???

JA = 2.73 open in a 1973-74 Nova or 2.56 open in a 1975 Nova
G = Detroit Gear and Axle assembly plant
178 = assembly date, 178th day of the year (June 27th or June 26th of a leap year)
4 = ???