Hi Jason,
Sounds like you have a RH 10 head, M2A mid year change up.
1974 was the year they started major changes to the 850 line, as with any manufacture incorporating new parts into assembly lines means old stuff has to go first, hens the mid year change up. The 30mm port isn't a design failure, its better enginerring for street use, and Norton realized that. In the world of induction enginerring ( heads & intakes ) having smaller intake ports and tuned intake manifolds results in broad power curves and thats what you wont for the street. Having peaky race style power curves suck on the street going from intersection to intersection. You asked about velocity, I hope this dosen't come out to technical but this is what we've leared over the years. There are two velocities at work here, port and swirl, tumble. Port velocity is the design of the intake track 28.5, 30, 32mm, size is the deciding factor. Swirl and tumble is the measurement of velocity entering the combustion chamber, swirl is for 2 valve heads and tumble is for 4 valve heads. The trick is to maintain velocity and swirl in the desired rpm band for the application. We have enough experience know that one can predict real world performance from what is observed on the flow bench. I hope this answered your questions.
Johnny Rocket