According to ISDRA Sand Dune Guide, Glamis elevation ranges from 200' - 500'. When we were there on Jan. 03, 2009 the density altitude ranged from 200' @ 7:00am - around 1500' at the worst during the day. We use density altitude for all our tuning and it takes into account: temp, relative humidity and barometric pressure.
You will also have to richen your air screws, pilot jets, needle position and of course your main jets. What type of carbs or jets are you using; the hex head jets or the round style?