as far as i know, it is quite difficult to get aftermarket gears for the borg warner 9 bolt rear. The largest oem gears are usually what everyone runs in those and those are 3.45s. When i upgrade my SS rear i'm just going to a ford 9", larger choice of gears sizes, easier gear swaps(3rd member) and stronger than the 9 or 10 bolt GM rears.
Also with your 10 bolt - 9 bolt swap, I am pretty sure all OE 9 bolts were disc brakes, and very few 10 bolt had those, in that case youd need to change the proportioning valve too, along with the gears for the speedo drive so your speedo will be correct.
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