I won't get into the question of performance, but to answer your question, I have modified several mufflers to fit engines with holes that didn't quite match up. The only question is if there is enough edge distance. Simply drill and tap the existing holes for the next larger size screw, Then screw in aluminum screws of the size you tapped using loctite. Cut the alumimum screws flush on both sides of the flange. You have now plugged the existing holes. If the muffler uses through bolts, layout and drill new holes to fit the engine. If it is tapped to recieve screws through the engine flange, layout, drill and tap new holes to fit. The job is simple if you have a drill press to assure the accuracy of the machine work but can be done freehand if you take care. I don't have a MACS in the shop now, but the picture shows an in-cowl muffler modified to fit an engine with a closer screw pattern. You can see where the original holes have been plugged.