[914Ghost]

I personally like dellorto's, Webers work great- but my 914 runs like its fuel injected, absolutely smooth.
I would run 40mm webers on a 1600 with 30mm Venturis. I think the Vent's in the Zeniths are 28mm.
The reasoning behind venturi size (diameter) and airflow is this:
The airflow into the head draws fuel from the various ports in the carb- starting with the idle ports and then to the "power circuit'. These overlap usually- the car will run off Idle AND power circuits much of the time.
If you keep the vacuum constant (same throttle plate opening) the size of the throat (venturi) determines how "rich" the mixture is. A smaller venturi with engine speed say at 3k will effectively cause the mixture to be similar as a larger venturi at 3800rpm .. if vacuum is constant more air must pass through the carb per time as the venturi size decreases which is equivilent to increasing engien speed. ..? See...don't quote my numbers - its just for thought.
So you can get away with running 48's and 40mm venturis but you'll never produce enough 'vacuum' to run in the correct mixture range in the carb. You can laugh at the weirdo's who covet the 48 IDA webers and run them on their 1700cc engines- they're useless.
My stock 914 has 40mm Dell's and 34mm Venturis- those are a teeeny bit big, but it runs perfect, just lacks a little low end- better with 30's.
Any 1.6L VW / Porsche can run a 36-40-or 44mm Weber or Dellorto carb setup and be tuned just fine (better than stock). For a stock engine the 40's are great
just run a 28 to 32mm venturi depending on your cam.
Weber's are great carbs, but they're a little noisy (throaty) and require a tiny bit more upkeep than Dellortos- and I've found the 'feel' of the Dell's is smoother.
I'll shut up now...
Bob O