There are mechanics and there are Specialty shops - anybody that knows the 350Z or G35 can also work on the FX35. Specialty shops might be a little more expensive, but you are paying for the fact that they see the same cars again and again, so they see the common issues with the VQ motor and also have connections for the best value fixes. I used Powerfab Automotive, outside of Houston for a lot of work on my FX when I lived in TX.
The fuel gauge issue is pretty well documented and has two causes;
1) the fuel level float arm inside the tank has corrosion at the hinge point and stops reading a consistent level
2) there are two fuel sender units in the tank - one left, one right. They average readings together to give you the level. One or both could be going bad... and they are not the cheapest thing.
