IMHO the perceived superiority of the single carb head is caused by the smaller ports, not the single carb as such.
One of the loveliest Triumph engines I've sampled was a TR6 to which I fitted a Webco twin carb conversion last year.
While I liked it with the single carb, I loved it with two. Beefy power all the way.
Like Bill, I like the long rod 650s better than the 750s but the British weeklies of the day got more speed out of the TR7 than the T140.
Putting a single carb on a T140 head will probably end in tears though.

Yes I know I'm banging on about the small port virtues, but I haven't come up with this all on my own- I stumbled on it when I built a fearfully quick TR7 a long time ago, and I've read John Healy and Pete R relating their views on it. In the Norton community it's a well known fact that on the 850 an RH10 head the 30mm ports makes for a much beefier midrange than a 32mm port RH4 head. And so on.