Tire and axle width ?

[2Tall]

Hi guys. I've been lurking on here for a good while now. Just now getting around to making my first post. My uncle has a 47 5 window chevy short bed that he plans on starting on soon. A couple of weeks ago at the street rod nationals he bought a super slam front and rear suspension from Scott's hotrods. So now he needs to find a rear end for when he starts on the rear suspension. He is planning on running 18's and 20's on the truck, so I'm wondering how wide a wheel he can run on the back without narrowing the factory frame? They will probably be a deep dish wheel, so I was also wondering what width rearend to use? Any info or pics of your trucks would be great. Thanks. Kyle