What part of Yosemite are you headed to? I was there 2 yrs. ago and drove thru this year and there are plenty of trees to hang from. My WB straps where plenty long for some large trees in Little Yosemite Valley and in the Backpacker camp.
Elevation effects different people different ways so its hard to say how it might effect you. I'm from NC and I have not had any problems.
You should take extra straps and a good down coat. I was in Kings Canyon in August it was 110 degrees in Fresno and dropped to the 50's up in the park.
Have a good trip, the views are awesome