Jeez Mike carby mate, not carbies. This isn't a high-tech free-revving motorcycle engine, this is a 70s-spec 1200cc Datsun boat anchor

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If you want carbies you'll need the aforementioned 180B SSS motor (or 200B SSS engine - but they're in rocking horse shit territory).
You might be better off cutting your losses, checking exactly how much room you're got and see if you can squeeze a ex-Jap wrecker 1600cc or 1800cc donk in there. When I was a young bloke they used to import these sort of things in droves from Japan, not sure if this is still done (it was a loooong time ago).
Either way you're going to have to learn to polish a sizable lump of manure. Good luck with it, seeing it cost you nothing should be a bundle of fun (just ask Mike about the GSX saga).
Ok, here's a late edit. The truly venerable A12 has been around for a long time, nearly as long as me. There was a twin-carb version in certain models of various Datsuns, but as far as I can tell, the closest place they were sold was NZ:
In New Zealand, a special edition Datsun 1200 SSS 4-door sedan with twin side-draft carburetors and other racy features was marketed
The original A12 gave a blistering 69HP, the twin-carb version 83HP. I'd go for one of these, get on the equivalent of the Trading Post in NZ and start hunting

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More reading:
http://en.wikipedia.org/wiki/Nissan_SunnyGotta love Wikipedia:
http://en.wikipedia.org/wiki/Nissan_A_engine and
http://en.wikipedia.org/wiki/Nissan_VanetteInteresting info about engine fires and recalls on that last one

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Mick