You could do that. As for the prudence of it, I'm not sure how the flow characteristics of the ITR muffler differ from the GReddy SP muffler. The muffler shop may even have a 2-bolt flange that'll match the GReddy B pipe so you may not even need to cut the flange off the GReddy axle-back section. That way, you can always go back to the GReddy, or sell it off if you don't want to keep it around. If you can get an ITR axle-back for cheap, and then have a muffler shop weld on the appropriate 2-bolt flange, and extend it where it needs to be extended, that'd be cool, and you wouldn't need to hack up your GReddy axle-back for the flange.