I do belive that for a rear axle, late 1970's Impala's came with 350's ( virtually all 1970's years ) or 305's ( '77 up ) and '79 up for the smog wheezing 267 ( which, I find out can be built up to a little somthing, now that I put 1 through the shreadder when I had the Impala parts car junked( Back when I realized hotrodding my '83 Benz would be an expensive project( Not that my '51 isn't expensive ), I'm pretty shure that like all Impala's up until like '80 or '81 had a posi rear end as an option, I'd also be willing to bet that it is near the same width as a '48 Chevy. late 70's impala's go fairly cheap, to check for posi, jack both sides of the car up and spin the wheels when in neutral or if the owner will let you, a simple burnout will surfice, 2 wheel tracks= posi, 1 wheel track= you don't want it