In my humble opinion, most of the time more backspace is much better than less backspace. If you figure the 255x60 is 27.5 tall, and 8.8 tread (according to summit, for a e.t.street R) that puts the tire 3/4" closer to the front edge of the fender opening, and about an 1/8-3/16ths closer to the fender. Judging by your pics, looks like it will fit, closest spot at the front of the opening.
If it ends up too close, you can shim the front spring hanger back, using longer bolts. The rear in my 62 Savoy is moved back an inch, lots more than you will need.
