I would be running a NOS sniper wet system on my L36 Ecotec VX, probably run a 70 hp shot so need to retard timing by about 2 degrees, is that correct? Can Mace do the tune for me?

We can definitely provide a tune to suit the throttle body and adjust the shift patterns. We can also adjust the timing at your request, but we cannot guarantee the tune will be safe with NOS. How far you retard the timing depends on each engine setup and the quality of the fuel used etc. I would suggest a wideband O2 sensor and gauge:

http://www.innovatemotorsports.com/

monitor your AFR to keep it within a safe range. It would be best to have a full custom dyno tune done to suit the 70hp shot, if you let us know your location we will maybe be able to suggest a good tuner in your area.