I'd be more concerned about leaving my 370z gearbox change in the hands of a dealership that doesn't sell & therefore doesn't service 370z's. I can only imagine what the test drive will be like to make sure the gearbox is ok.
I don't trust many people to work on my car unless they are known good specialists,e.g. Horsham Developments, Abbey etc. I understand you have to use a Nissan dealer but I'd be doing a little bit more research and getting my car sorted at one of the better ones that sells and has experience of 370z's, even of it was miles away IMHO.
Good luck to the OP, fingers crossed for a positive experience and outcome.