We do this on a regular basis and would need to know:
What donor vehicle furnished the rear end?
10 bolt, 12 bolt, Ford 8"/9", other?
What engine is in the vehicle?
Ring gear and pinion sets are not terribly expensive and save time and expense over a rear end swap.
Final drive (.76 5th gear) ratios:
3.42----2.60
3.73----2.83
3.90----2.96
4.11----3.12
For a 6 cylinder GM used a 4.11 or a 3.90 rear gear with the .67 BW OD trans. This gives good lugging power for loaded starts yet low rpm cruise speed in OD.
If a high torque output V8 then the 3.73 is fine. If a big engine and just a never loaded cruiser then one can go down into the mid two's for a final drive.
