Would SU carbs from a L24 be any good on a L20. How would they compare with a set of triples for performance, or would they be a waste of time.

I have a 240 with SUs and it goes very well, so I assume they should flow enough air for an L20.

Has anyone done this?

The Datsun roadster (2 liter=U 20 motor) uses HS6's. They are virtually identical to those of the Z.

The L-20A came with the 38mm carbs where the 240Z came with 46mm carbs. By stepping up to the 46mm carbs popular opinion would suggest you'd lose bottom end torque.

As far as triples are concerned, I think it would be very easy to over-do things on that engine.