The problem with my Hitachi/SUs was that the holes in the side of the body that took the butterfly valve shaft wore oblong after 80K miles or so, and after that you couldn't set the mixture stably, or precisely synchronize the carbs.
The Weber aftermarket fit was, as I recall, pretty popular.
Other mods to the early 240s were
1. A bigger rear anti-roll bar
2. Adjustable plates for the top of the McPherson struts
3. Remounting the differential slightly rearward (production change to later units)
4. Factory available 5-speed with better, Porsche-style, synchronizers
5. Wider wheels--you had to get the right offset to keep things from rubbing, though
6. Headers
7. Air conditioning. For the 240Z, this was strictly aftermarket dealer-installed. Mine was the first one done in town, and they made kind of a mess of it.
8. Cosmetics--Mag wheels, front chin spoiler, headlight "sugar scoop" covers, driving lights, etc.
I did only a handful of these things to mine.
my fave option was the small block 350