Looking to install original rally rims on 70 Velle

[younggun]

I am looking to install the original set of rallys on my Chevelle. I measured the backspace on the rim and I am showing a 4" backspace. Is this correct? I see a lot of people in the forums stating that 3.75" is the usual original backspacing. I am looking to run a bigger tire in the back so I am looking into what the biggest tire is that would fit in the rear wells without any mods. The rims are 15 x 8. I am going to run a 225/50/15 in the front due a 3" in drop. The rear suspension is stock height. I was hoping to run a 275/50 on the rear but I am not sure if that will fit. Any help would be appreciated.