I have no idea what brand/manufacturer my muffler is, my engine is about the same specs as yours but I've got a 2.25" system back to the muffler - it matches the bore of the extractors. I got friendly with the exhaust bloke and told him what I wanted out of the car, "I want it to flow as well as possible, and I want it quiet as possible" - he showed me the same muffler off the shelf after he fitted it, if you look through it, you can see a semicircle of daylight at the other end. I wouldn't call it quiet though.... when I pulled up at the mini experience launch after idling in, a bloke came up to me and said "do you have something special under the bonnet of this?" I said "nah, just a warm 1275", he said "oh, I thought it sounded like a rotary"
moral is, go to a small exhaust shop who look like they know what they're doing when it comes to old performance cars and tell them what you want
) I had to put an extra hotdog in to drop the noise a bit, It's still loud just doesn't set off alarms now 