Good point on the D manifolds. Of course the rear one wouldn't fit on an earlier motor because Ds had two front heads. Doh! Dunce cap time! Hence the welding. The front one would be fine but, yes, you'd have to alter a rear manifold to fit a B or C rear head. Before you do, though, it's worth asking around because older Vincent owners with boxes of old parts sometimes have things like this lying fallow...
PK