A 1HZ with an aftermarket turbo is not eligible for the EPA exemption ordinarily granted to 21+ year old cars because any modification to the original motor is considered "tampering" with the emissions system, and is a violation of the Clean Air Act. Technically, it would not be legal to import such a modified car unless you bring it into compliance with EPA regulations that were in place when it was manufactured (or you de-modify it back to original to regain the exemption).
That said, it's not the kind of violation that's likely to get your car crushed--the U.S. Department of Justice has bigger fish to fry--and US Customs might not even catch it.
I'm going to guess you have a truck already bought and on its way? If so, I can't say I'd recommend calling US Customs or the EPA to give them your name and tell them about all the illegal things you're about to try to get away with. That can only end badly. If the truck is inbound, just ride it out--keep moving forward with the process until you succeed or something stops you. Don't mention the modifications. It's been said that non-disclosure is not deception (lying is deception, though, so never lie to Federal officials). Chances are it will go fine and you'll get your truck*.
*Your truck will not be legally in the US, of course, but at least you'll have it. You can always return it to stock configuration after you get it to make it US legal, or you can just live with the small risk of Federal Agents raiding your home at dawn. Believe me, there are guys on IH8MUD who have far more legal exposure and I'm pretty sure they sleep like babies.
