Use a period sometime!
The 3.42 gear was very popular in those cars. It's a good all around gear. Many late '70s automatic Z28s had those gears with the Traction Lock differential (posi). If you look around enough you can probably find one for sale. If you decide to go with new parts I recommend the Eaton Posi, Yukon or US Gear and a good installation kit. Randy's Ring & Pinion or Tom's Differentials are some of the best places to get these parts.