Just bought a 76 280Z with two ?Weber? carbs on it. Is this right? All the write ups I've see indicate that Datsun/Nissan went to FI effective in 1975. How would I be able to tell if the car originally had FI and someone switched over to Carbs? Would the VIN have this information?

well i would be running a stage 3 cam and a MSA header. would SU's or EFI be better (say if i had larger injectors and a 240sx throttle body.)

Stage 3 sounds like a lot of cam for EFI, maybe even too much for SUs. I'd ask the cam grinder what they recommend for use with that cam.

Keep the round tops, and for a cheap cam, try the 260 stock cam. check the numbers, it's a tad bit hotter.