Rearend and tire combo question.

[whytry]

On my 68 Camaro I have 18x8 with 4.5 B/S and I have a 255/45/18 on it and it is very close to the outer part of the fenderwell and there is adequete space on the inside to fit a 275/40/18 back there but if I leave everything alone it looks like it will rub on the outside.... Now I am getting a new rearend built and I have the option to shorten my 9". I was looking at shortening it 1/4" - 1/2" on either side to get the clearance I need. Does this sound right? From what I have read in the past my wheels should have been 4.75 B/S, not 4.5. If I shorten the rearend by 1/4" - 1/2", it would be like ordering wheels with 4.75 B/S right? Also giving me the clearance to fit the 275/40 I want. Any help appreciated...