Thats incorrect. The 25yr rule applies to trucks with their original engine. If the engine isn't original then the truck isn't eligible for entry. Td5 and tdi engines aren't epa approved and aren't eligible/legal no matter what.
I agree that as a practical matter the 25 year old exemption applies only when original drivetrain is intact. However, I was under the impression that the EPA only regulates the importation of vehicles, not parts, per this EPA document:
http://www.epa.gov/oms/imports/quiktext.htm
which says, in pertinent part:
B. (2) Engine not in Vehicle or Chassis
EPA regulates the entire vehicle, not individual parts, for cars, light trucks, and motorcycles. If an engine is not installed, and is to be used in a car, motorcycle, or light truck, it may be imported as an automotive part. No approval or Customs bond is required by EPA.
So, I think it is fine to import your 300tdi or your TD5, so long as it isn't installed in a chassis. You only commit a crime when you install such an engine in a vehicle that IS subject to EPA regulations (which is 21 years old, not 25 years like NHTSA). Put the non-EPA certified engine in an exempt vehicle, and you should be fine.
I admit that just looking at the EPA Form 3250-1 can lead to some confusion on the issue, but that is why the EPA publishes documents to help resolve ambiguities that may come up if you are just relying just on the forms.