Let me state that I do know the rules. It is my job, I work for the EPA.

Important information on engine switching is that engine or engine chases designs are "certified" (approved) by the U.S. Environmental Protection Agency (EPA). This "certified configuration" involves all emission related parts and drive train components for that particular model and model year. When making an engine switch, you must make sure that the switch results in a vehicle with a certified configuration of the same or newer model year as the vehicle chassis. An "identical" engine switch, with reconnection of all emission related parts, is the simplest situation and should be acceptable. If you are attempting to put an engine of one vehicle manufacturer into a vehicle by another manufacturer, it is generally not possible to obtain a certified configuration, without a great deal of work (i.e., potentially replacing the engine and all it's components, transmission, drive axles, exhaust system, etc.) and accompanying expense. If one wishes to replace a gasoline engine with a diesel engine, or vice versa, the vehicle chassis in question must be certified with both engine types. Then, if the exchange is one with an approved configuration of the same or newer model year, with all components properly installed and connected, the switch should be legal. For all cases, the appropriate source for technical information regarding the certified configuration of a vehicle of a particular model year is the vehicle manufacturer.

You can find out anything you need to know at:
http://www.epa.gov/Compliance/resources/policies/civil/caa/mobile/engswitch.pdf

By the way, gutting your catalytic converter is also illegal.