Yes to get the deep dish look you will have to narrow the rear end...how much? That's up to how much dish you want. The best way to approach it is to buy the wheels you want, then mock them up (with tires on) and measure for the rear end. I am sure plenty of guys can tell you what they used numbers wise...but every car can very a little.
On a 68 Camaro a 315 tire on a 11" wheel fits nice with the DSE Deep Tubs....if you are willing to trim or roll the outer fender lip you can get a 335 on a 12" wheel in there. FWIW, I have a 335 on a 11.5" wheel on my 68 with trimmed lips.
Oh, and welcome to the site!!