they arent carbs, they are ITB's...
A lot of people have done this, usually using the GSXR (also called Hayabusa) ITB's (42mm).
Here is another guide:
http://en.wikibooks.org/wiki/Nissan_240SX_Performance_Modification/KA24DE_and_KA24E/ITB's#Custom_ITB_Setup_For_KA24DE_using_GSXR_750_o r_GSXR_1000_Throttle_Bodies
Note, there is only a 5HP increase, and if put a megaquirt on the stock plenum and manifold you would also get a 5HP increase, because it doesnt have the emmisions control, so make you mind up about that one...
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