The 84-86 USA 700 nighthawks had 31mm carbs, the 91-03 USA 750 nighthawks have 30mm carbs, and the 84-86 750 Nighthawks that were not available in the USA had those 34mm carbs.
These all look about the same, and look like the same type you're considering. The jetting can likely be made to work too.
The only issue I can think of is the spacing from one carb to the next. It may or may not be a direct fit.
Cheers.
-A