The ideal rear axle would have been 3.08 with either 14" (70 series) or 15" (60 series) tyres, which is very close to how it was in the Commodore from the factory. However you mentioned that your engine has a 'mild' cam & extractors, this will push the available torque further up the rev range.

A stock 5.0 EFI will cruise happily at 110 km/h @ 1800 rpm, but your combination will struggle at those revs. You will need to go to 3.36 or even 3.55, depending on how 'mild' the cam profile is.

Dr Terry