The atmospheric pressure that you SLEEP at is actually more important to acclimatization than the pressure that you ride at. Many of the pro riders actually sleep in hypoxia tents that you see advertised in Bicycling Magazine. Getting to the high altitude as soon as possible and sleeping at that altitude BEFORE you do any exercise will help a lot. Even one night really helps.

When I hiked to Mt. Whitney, I got such bad altitude sickeness at 12000 feet that I didn't think I would make it to the top. After spending the night there at the 12000 level, I was able to make it to the top the next day. Huge difference in how I felt!