When it comes to differentials you can go about getting the stock diffs from that years which i think were 3.08, 3.23, and 3.42 for the bigger ones. But if you wanted to go all out they sell 3.73 and 4.10 rear axle assemblies, which will definitely make a difference. I own a 78 Firebird formula but im putting a 400 in mine, with probably 3.73. And as far as exhaust if your willing to do the work putting a true dual with flowmasters super 44's will make it scream. If you could send me a message with a little information about your 78 firebird.