According to the GM Heritage Center
1957 Chevrolet Truck Data PDF the 3106 (Suburban Carryall) was either equipped with a standard 3.90 ratio gear set, or an optional 4.11 ratio gear set.
The difference can be noted by checking the passenger side differential pumpkin casting where the ID info is stamped. The standard 3.90 ratio unit ID stamping will begin with the letters "AF". The optional 4.11 ratio unit ID stamping will begin with the letters "AG".
As far as which ratio rear axle will work best with your engine and T5 transmission, that depends on both the engine being used and the T5's OD ratio. More details needed please.
