i bet you have the 4.10's now, so if you went with 4.56 you'd be spinning about 11% faster at 80 than you are now, and if you went to 4.88 you'd be 19% high rpms.
He said he has a V6 and auto, that combo would not come with 4.10 gears, it would have 4.56, possibly 4.88 from factory.