Twin-tube mufflers (actually 1 pipe into 2 for looks) are still made and available. I think you're right that Ansa is no longer in biz, but you can get essential clones from Vic British (I think under the 'Monza' name) and Motorsport Auto ('Targa?'). They both run about $140-$150 bucks for the cat-back system (muffler and pipe from the catalytic converter back). I got one of these from MSA -- one setup fits all '75-'78 280s -- but have not installed it yet. I got it because I saw a Z car with one and also liked the way it sounded.
FYI, The more power-hungry contributors to this forum tend to sneer at these bolt-in systems because they use only 2" diameter pipe. 2.5" is considered by many to be the minimum 'right' diameter to use for increased power. My take is that big pipes don't do much until you start doing mods to the intake and fuel delivery, which I'm not willing (or financially able) to do. Personally, I'm pretty happy with the way my stock EFI motor performs and since the stock exhaust is about 2", I'm confident the targa system will work fine for me.