Hi jab,
You have two relatively easy options for mating flex track to FasTrack, Atlas and Gargraves. Both will match the height of FasTrack when laid on top of 5mm cork roadbed. To join either you will need FasTrack tubular transition sections (Lionel #6-12040). Remove the tubular pins from the transition sections and use Atlas transition rail joiners (Atlas #151-6095) or Gargraves transition pins (Gargraves #801). Neither system is a particularly good match to FasTrack visually, but Atlas would be closer. Personally, I would choose Gargraves because it is much easier to work with than Atlas flex.