I would suggest searching the archives some. Everyone has a slightly different opinion of a "good fit". Fit covers both interference and appearance.

I would not expect there to be any issues with 255/60/15 on the 15X8 4.5BS wheel. A 275/60/15 on the same rim may be a slightly different story, depending on your vehicle ride height and how much suspension compression could occur during extremes of your driving.

Search for "275/60/15" in the Q&A to see an old post of mine where I expressed my opinions of fitment on a 70 Cuda.

My suggestion is to gather various opinions, look at many pictures as possible, and take lots of measurements!

Good luck!
Dave