I'd recommend you do a HPI check yourself and then offer that as proof that it is not recorded as a Cat D.
I also strongly urge any potential buyer to do their own HPI check, and get an independent engineers report.
I understand you are probably just wanting to get your money back that you put into it. But regardless of whether it is recorded damage or not, the car has been acknowledged to have suffered enough damage for an insurance company to write it off.
As such, it should be classed in any buyer or sellers eyes as a Cat D car. Which means you will not get market value for it.
And as for exporting your car to the US - good luck with that. It's is a complete pain in the arse and will cost you a fair sum. The 350z is pretty cheap over there, you would be better cutting your losses here, selling for £6k, and buying another over in the US.