OK, Michael as you now know, fitting of a replacement flywheel wronly appears to be an occuring issue with some garages.
This should not really happen as firstly, most replacement flys do come with illustrated fitment instructions and common sense must prevail.
With the Fidanza Fly you do have to be careful as there are two options for fitment.
One is clearly marked FRONT for Maxima & Altima.
The other is clearly marked REAR for 350Z, G35 & G37.
On other applications available purely for RWD such as 350Z, G35 & C37 there is only one fitment application, so wrong fitment becomes almost impossible.
This is the application that was sent to your garage and how they managed to fook up is totally beyond me.
There is a male pin on the crank which slots into the female hole on the fly. simples.
This has all to do with the CPS (crank position sensor) picking up the correct signal from the fly.
It is so important that these types of cars are only worked on by specialists, however it is to your garage's credit that they have held there hands up and admitted the error of there ways.
I have lost count of the number of garages (including Nissan Main Dealers) who have phoned me after fitting one of my fly/clutch kits with the usual: "I wonder if you can help me, but I have just fitted one of your clutch/fly kits and the car is not running right."
The best advice that I can give you, in this instance Michael is to wait until you get the car back from them and judge then on the end product.
If there are still issues then send in the cavalry.
There is also now the added complication of the DE fly being different from the HR fly.
I know of a well respected trader on this forum who ordered from me a HR fly and fitted it to a DE motor, of course the car would not start and they phoned me with there concerns and after a short conversation I was able to offer the solution.
Moral of the story is that we all can make mistakes as every day is a school day
Alex,