Probes should be at the hottest part of the vivarium that the Iggy can reach. If you place all the heat sources focusing in 1 corner and it warms upto say 90F, then the opposite corner will probably be in the region of 80-85F depening on your airflow - which is about right.
All you need to control with the thermostats is the hot end of the vivarium. The rest of the vivarium will naturally be cooler. So I would concentrate the heating and probes at one end of the viv. Make sure the probes are in the hottest point of the vivarium.
Once set up you need a digital thermometer, with a probe, that displays both internal and external temps. That way you can put the probe in the hot area and the thermometer in the cool area and get a reading for both on the screen. Then you will know if the temps are acceptable before Ig goes in.
I would agree on a "regular" sized vivarium but this is 6'x6'x4'. Put the probe 6' away from the heat source - yeah you'll get an ambient temp but you're gonna have to make sure the Ig can't get too close to the heat source or he/she will get cremated unless there is little air-circulation, in which case Ig will get respiratory infection instead because of high humidity and stagnant air.
Good air circulation and 70% humidity will mean the Ig won't get respiratory problems. If the probe is 6' from the heat source in these circumstances then the temperature within 1-2' of the heat source will likely be extremely high - more like Monitor or Tegu high than Iguana high. Don't forget your Ig is going to be aiming to get to the highest resting point possible, hot air rises.
I'm just going on my experiences, you may find things work out differently.
good points,oringaly was set one probe on hot side stat on the 2 bigger heaters,one on the cool side stat on the 2 smaller heaters,dimmer stat on the basking light,air vents top and bottom of back,and maybe a fan