You always have to make that modification when using larger tires. Most people just do it right and section the rear quarter and stretch it with a patch panel, like this.
That's what was done on my old '69 Dart that I built to race NHRA Super Street class years ago. Rules said the wheel opening could not be raised but made no mention about stretching it. It was a tubbed back-halved car and we stuffed 14x32 slicks under it. Looked great!