yep...
Puting a 300zx TT engine in a 240sx is way too much of a big project, and would cost ALOT.
And SR20DET can be had for a reasonable price and the 240sx chassis AND tranny are exactly the same as the sylvia so it all made to sustain that power.
About smog, hmm, I think SR20DET's are OBDI thats one problem, it wont pas visual THATS for sure, for emissions, well, at low PSI with cat converter etc.... It should be ok, but Id have to check, maybe 240sx.org has info on that.