If using a 67 or older housing it has the same width as a 67 Camaro, The 68 and up chevelle housing is the same width as a 70 and up camaro....I believe that info is correct. By the way I have a 67 monoleaf 12-bolt for sale, but it is not a good canidate for modifications.....to pricey and it is a traction bar rearend.
A cheaper alternative to modify would be maybe an Impala rearend, they do have bigger brakes, but I would go to ford ends for the axle retention....My.02 cents.