The Z car SU's will work on a four-cylinder L-series, but they won't bolt-on like the 38mm SUs. The manifold will require some "opening up" of the carb throats to match the increased carb bore. Before we even get to that point, however, what kind of motor are you going to bolt them up to? Experience (not mine, but Jason Gray, Kelvin, etc.) has proven that the bigger SU's aren't really suitable for a street driven car unless the motor is bigger than an L20B, or you run at WOT all day long. The big SU's are certainly too big for an L16/L18. See this site for more proof: http://www.mgbmga.com/tech/mgb19.htm
That said, you still have options. First, if you buy the Z SUs from the junkyard, you'll have to go through them to make sure they're still in good running condition. If you unbolt the carbs while in the JY, and then try to wiggle the throttle shafts...if you get too much play, I would suggest you pass on them. They'll require at least another $100+ to get them functional.
But, say the carbs seem in good shape. You can buy the carbs. You will, next, need to get a hold of a manifold for your 4cyl L-series. Obviously the Z manifold won't work. New manifolds are available from Nissan for $150, last time I checked.
Now, say you have the carbs, have the manifold, and still want to bolt them up, even to a smaller motor? There are only two main problems left. One, you'll have to fabricate a linkage because the Z linkage rotates opposite of a 510. You can fiddle with the linkage, possibly buy new linkage from Nissan, or switch/fabricate a cable linkage setup. The other main problem is correct needles to adapt the Z sized carbs to your smaller motor.
Nissan never did this, but I have a book for Brit SU equipped cars. Lots of Brit cars with smaller motors came with dual 1-3/4" SUs. You'll have to switch needles and dome springs, but it can work. It just won't be as responsive around town as the smaller 38mm SUs. Why? See Theory below.
Sorry, I guess that's kind of long. You can make them work, but they won't bolt up. If you get them, let me know and i can give you some specs on needles and springs from my book. If you want to see what the other fellows have to say about the Z SUs, you could search the bluebirds archive, or possibly talk to Jason Gray, or Dan McGee (though he went off list...he plans on moving from the smaller to bigger SUs. He's got a pretty hot L20B and is happy with the small carbs.)
Why don't Z-car SUs work on smaller 4-cylinder L-series motors?
Both the 38mm and 46mm carbs should be able to setup to provide the same venturi air velocity. This is not the problem as I see it. Lets say you are accelerating a L20B full throttle from 2000 to 4000 RPM. Do this first with a small 38mm SU setup, there will be upward movement of the slide piston and fuel needle to meter fuel to this changing volume of air. Lets say the needle of the 38mm SUs rises 20mm during out 2000 to 4000RPM test. Now do the same 2000 to 4000RPM acceleration test with a set of 46mm SUs. The 46mm SUs with larger diameter venturi will need to raise the piston/needle assembly less than the 38mm setup in order to flow the same amount of air. Lets say the 46mm SUs needles only raise up by 15mm. In each test, metering the same amount of air, the larger SUs will rely on less upward movement of the needle to meter the fuel so there are fewer "fuel flow mapping points" on the physical surface of the needle and accuracy of the air/fuel ratio is reduced . (think of the physical surface of the needle like the electronic bits of information in the fuel map of an EFI brain).
The overall situation is also analogous to the factory sizing of EFI airflow meter, where a small airflow meter is used to provide more accurate info to the EFI brain for better emmisions control.
"I did a 46mm SU adaptation a couple of years ago. If I remember
right, the hole in the 240Z phenolic spacer was actualy closer
to 48mm so you are going to need to hog out the bore in the 510
manifold to get it gasket matched to the Z spacer. The main probelm
I saw with grinding the manifold hole this big is that this leaves
little gasket surface on the manifold flange side and makes it
more prone to vacuum leaks (especialy considering the SSS manifold
only has provisions for using 2 of the 4 carb flange bolts).
I doubt that it is necesary to increase the manifold hole all the way to 48mm or even 46mm to get the full benefit of increased airflow, and keeping the manifold bore size slightly smaller helps to preserve velocity. The carb bore is 46mm at the throttle plate but the throttle shaft presents a restriction to airflow even at WOT so you are not going to get airflow equivalent to an unrestricted 46mm hole through the carb (nevermind that the jet bridge and intake side of the carb also present restrictions). What I did was to grind the manifold hole out to 42mm and then modified the phenolic spacer to taper from 48mm at the carb down to 42mm at the manifold to match the manifold hole. I didn't grind out a 38mm spacer to do this, I filled in the ID of the 240Z spacer hole with fibreglass strands and epoxy, then laid up the wet epoxy/firbeglass to sort of form the desired taper ID to 42mm. Once it was dry I grinded and sanded the fibreglass taper ring to final smooth configuration."