71 Duster rear wheel and tire combination help

I'm currently rocking 225/70R14's on all four corners of my duster. I really like the rallye rims, so I thought about buying some 15x8's with back spacing of 4-5/8" for the back this spring, My main question is what size tire will fill the fenders up the most without rubbing. The springs haven't been relocated and I haven't rolled the fenders. I was thinking a 275/60R15 would fit, but I'm not sure if it would be too wide. I'm just looking to tap into everyone's knowledge and see what they use on the back.

It looks so puny now, I'm just trying to achieve a beefier look!