I think in some states such as California it is illegal to drop an engine into your car that was made before your car was. (for example, you couldn't put a '86 2.3T into your 87 Mustang.) I don't think they can really tell though.
As far as emissions(smog check) go's the 2.3 Turbo mite be illegal.The 5.0 engine would pass emissions tho.
yeah, it might be illegal some places frodifferent reasons, but at least here in VA, its perfectly legal.