Minitubs are used to gain clearance on the inside of the tire....ie. towards the frame rail. This is mainly used for wheels with more backspacing, thus allowing you to "tuck" the wheels in under the quarters. If you are experiencing minor rubbing you can roll the lip on the wheels well opening to give more clearance just inside the quarter panel.

I've got a 69 firebird with 15x8 and 295/50/15 on the rear with no rubbing problems. I rolled the outer lip and my back spacing is right at 5 inches. So, those wheels MAY work on your car. Measure the back spacing, the measure the distance from the inner fender to a straight line across the mounting surface of your drums. You should be able to fit a 255/60/15 tire on a 15x8 in under the quarter panel with no problem depending on back spacing.