rearching wheel opening???

I'm wanting to open up my wheel opening in the back of my 69 Barracuda. I have 275/60/15's on the back and I don't like the way it looks. I love the wheel and tire and wanna stay with that size but the front part of the wheel opening is close to the tire- not close enough to tough, I just want a more round opening. Any body do this?? Have any pics or tips?? The car is in primer and body work is slowly being done I'd like to do it now