One horsepower-increaser fix that was common in the late '70s- early '80s was to remove the 'final baffle' in some ANSAs. This was a steel washer tack-welded across the perforated output pipe inside the case, which cut 2-3 dB from the exhaust noise... and about 25 bhp from the engine! Its one of two differences between a GTS ANSA and a 'normal' one. Note- not all ANSAs were built the same- there were at least 5 different variations on Panteras over the years. Some were baffled, some weren't. Easy enough to check with a long screwdriver.
If you remove the muffler, cut out the circular shim inside the inlet plus remove the washer-baffles, now you've got a faux-GTS/GR-3 muffler with a much less restricted 2.25" OD inlet instead of 2". For several hundred $$$ less than factory parts (real GTS mufflers had 2.50" OD inlet pipes w/bracing gussets around the inlets and no washer-baffles). To remove the baffles, one rams a piece of pipe up the chrome tip and smacks it with a hammer to break the washer-tacks. Some called this the 'broomstick power-fix'.
What none of us thought about was, now you've also got chunks of steel rattling around inside the mufflers unless they got stuck somewhere. I cut a small hole in one side, bent it up enough to get a magnet inside and fished the washers out from both mufflers. Then bent the cut back down and rewelded them. Bingo- rattles gone and wife Judy was happy!