The rubber manifolds are are slightly different, as evidenced by the fact that they are actually different part numbers for the early and later years (later years being 77-78). However, many people would be hard pressed to tell and are able to "make it work."
The actual intake manifolds vary with the existence of a vacuum sync port. Exists on early years, absent on PD years (77-78). On PD carbs there is a vacuum port on the carb body, non-existent on early years.
For you this wouldn't be an issue, if you put later carbs on your early bike, you would just have a set of vacuum port screws on the manifolds, and then also on the carb bodies. If it were the other way around, you would have no vacuum ports anywhere.
The jetting is different but it is possible to work as is. The engine is however mostly the same, so, this swap can be done.
The 78 carbs however, do not have a choke lever, it has a cable actuated choke. So you would also need to purchase a cable, and a bracket to mount it to the top triple, or fabricate one. The proposition of using the choke without the cable is not really possible (practically).
