I have an 81 Z with 3.73 gears and I swapped in a TKO 600 with the .82 OD rather that the .64. My friend has a 69 Firebird with 3.73’s and he had swapped in the TKO 600 with the .64 Od. On the highways here in Florida the .64 was great, but those highway jaunts were just from one exit to another, 99% of the time his ‘bird is street driven, and that’s were the .82 shines. Driving his firebird, it felt like there should’ve been a gear between final drive an the .64. Anything under 55 would seriously lug the engine. With my .82 setup, 70mph is 2600 rpm’s for me (255/60/15 rear tires). My point in all this jabbering is that you should consider how the car is driven before dropping 5k (I bought a whole conversion kit, and yes it fits PERFECTLY!) IMHO, I’d take the .82 over the .64..