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The factory put 255/60R15's on 78-82's, which are about 27" diameter. A 275/60R15 is about an inch taller, and a 295/50R15 comes in just a little shorter at about 26.7".
The 295/50 is a little over an inch wider in the tread than the 255/60, and has a section width 2 inches wider. The tread width shouldn't be an issue, but the section width might cause interference with the spring ends or parking brake cable brackets.
I personally wouldn't recommend wider than the 255/60s on stock 8" wide rally's though. The 255's are already reasonably wide for an 8" rim. You can certainly fit wider, but they will look puffy on the wheel and will flex a lot in corners.