I can see it's a dilemma.
Unfortunately it could go either way though, insurange companies WILL try to wriggle out of a claim if they can - i.e. they may say "its not a GT, so the policy is void" etc... Other import marques and variations are well known to insurance companies, so I don't see why they can't get the facts right for the 350Z!
By the way Ricky, have you tried A-Plan insurance? They are excellent for imports... I've been using them for years and continued to use them once I got my UK Z...
0845 071 1234 - http://www.aplan.co.uk - give them a go!