Are you convinced you need a 2 into one? I run single pipes, I am biased, but I can show you a graph that says it's better in the mid range than a Maney system with a PW3 profile cam (JS2 is PW3ish)!
And I reckon mid range is what you need on an outfit? And don't say 'a 2 into 1 makes better mid range and single pipes make better top end' that is a popular myth which the comparison of my system and Maney's shows to be wrong on both points. Search some threads on here, it has been discussed, you might want to look for inputs from 'Snotzo'.
Anyway, Nigel at NRP made it. And he has made many more. Nortons well catered for, just tell him the cam you are using.
On the noise front, the Maney system has always been noted as 'noisy'. Single pipes have the benefit of two cans to help with that. Nigel is pretty close to the racers and knows all about meeting UK noise regs.
From my perspective, NRP is 'on your doorstep'!
A word on primary length for a 1 3/4" ID pipe, you will measure '73/'74 works Nortons as 28", I used that on my '75/'76 build 850, and it works fine, but.....you need Norman White to tell you that the pipe you could see was indeed 28" long, but a further 2" went inside the megaphone
I can tell you Nigel's pipes on my 750 short stroke are longer than 28", however there isn't a clear start point for a Mega as in the old days, the pipe gets bigger in steps until it gets to the silencer.
So there you go, you now have 'anything from 28" to 37"' Bet you are glad you asked!