Well, thats a pretty obvious thing there. You come out of Canada with a truck that is going to be RHD, still have an English registration and papers and will be newer than the 1985 (current) import limitation. Thatll be obvious. I doubt that a USDM Disco with a non stock motor is going to be a blip on their screen. They will ask you for your ID at the border crossing, what you are planning on doing there and what the length of stay is going to be.
Lets not forget that the border crossings are like toll booths:
Read this link about tips for crossing into Canada. There is mention of registration and nothing else. I think that you are overthinking it. Do you honestly think that they are going to inspect your car for EPA adherence at the border crossing? If you have ID, answer questions honestly, give them a legit registration from your state that is for your vehicle that is operating in a safe and responsible way, why would they feel the need to pull out the EPA guidebook, research your specific vehicle and compare it to your vehicles current standard? Just about everything is illegal when it comes to the EPA.