Some of you could do yourselves good by reading the statutes. In general, they can be summarized as follows:
You cannot sell a vehicle that's been tampered with (exceptions exist, of course)
A motor vehicle dealer cannot legally perform emissions deletions without having a waiver signed for off-road use.
An entity cannot sell items specifically to defeat an emission systems (except for off-road use).
You cannot tamper with gasoline powered vehicles that are used on-road.
You cannot have continuous visible emissions longer than 5 seconds unless under acceleration, lugging, deceleration.
There are other things, but that's the gist of it. I've yet to read a statute that prohibits the on-road use of a diesel truck in which the owner has modified/removed emissions equipment. Not to say that one doesn't exist, but I've not seen it. There's been tons of discussion of this for years, but no one has ever seemed to find a statute that says if you (private person) perform your own work to modify your vehicle, then you are in violation of the law. I could be proven wrong, which I would actually welcome. I'm not against knowing the truth, even if it means I'm 100% incorrect.
There's no advice implied above, and I'm just referring to the statutes I've read (both federal and state). Your state might have different rules/regs, so that's up to you.
Note that this entire argument originated with the personal vendetta of someone who believes their privileges outweigh someone else's, which is the saddest and most arrogant position to take.