Some very interesting reading in that lot, especially the link you put up Musky. Interesting to note that Phil Irving recommended 76 degree as the optimum angle yet the motor he designed and of course was famous for started with a 47 degree angle and ended with a 50 degree. Maybe he wanted 76 but manufacturing or other factors meant they ended up with the 47.
I reckon there are several was of looking at all this. There is no doubt a parallel twin vibrates, they always have and without a counter rotating shaft, they always will. The only thing that can be changed is just where (as in what rpm) they vibrate at. I can see the advantages in these crank modifications but the big question is why ? You can go and buy a 270 degree crank (almost) vertical twin in the form of a TRX Yamaha, or there are lots of 90 degree twins on the market, Guzzi's, Ducati's etc etc. So why butcher up a heap of BSA parts to make something you can get easily. Maybe there doesnt have to be a reason ?
I'm a big fan of V twins, I have a 1000s Guzzi that I bought new in 94' and have done 86,000 k's on so far. Its a terrific bike but has one minor problem, the faster you go the smoother it gets. At around 5,000 rpm which is 100mph it is totally smooth, not the slightest hint of a vibe but its difficult to explain this to a Policeman ! Maybe its because I have a V twin I dont have the desire to build something out of a BSA. I think the comment " you would have to have money to burn" is very fitting as all this would involve a lot of machining, you would have to be very skilled and have access to a lot of machinery to do something like this.
So I guess what I am saying is leave the BSA alone and enjoy it for what it is, if you want a V twin, go and buy one. Just my opinion, mind you the "other half" says I'm not allowed to have one of those.............