If you want to cut it... cut it! Someone can always weld new rear flares on in the future. If the truck was a perfect all original truck uncut with perfect paint then I would think twice. Just like others have said. Find the center on the opening, tape the flares on put a screw or two in it, and step back and take a look. Mark the inside of the flare and cut with a body saw, Sawzall, or small cut off wheel. Smooth the edge and put a little pain on it to seal it. I used the supplied rivets on my flares and have had no issues in 14 years. But the flat head screws or bolts look good too!
