I believe the Pontiac hams interchanged with the Olds. Pontiac did have more ratios available, their 4.33 should be the one, IIRC. You can always pick up an aftermarket R & P. Bob
These people can help you out on tech, gears and parts for the 57-64 Pontiac/ Olds rear end stuff. link=http://www.fabcraftmetalworks.com/
Back many years ago I had a 1950 Oldsmobile model 76 (6 cylinder auto trans)that had a 4.30:1 rear axle. Someone told me once they were known as,"mountain gears" but couldn't explain why. This particular car came from Colorado so it may have been an optional ratio. When I installed a rather large later model Olds(475 cu.in.) I wound up welding the spiders because the single track would spin all day and a posi unit wasn't available. I have a 4.56:1 posi unit in my 57 Safari but those are Richmond gears.I'm sure you can find a 4.30 gear set for the 57 rear but it will require(IIRC)a spacer for the ring gear to properly set it up.