Not a problem. Just secure them so they don't roll around. The tanks will not expand... too tough for that, but the internal pressure will change. (Happens to be Charles Law of Gases...
http://scubatoys.com/education/charles.asp)
In a nutshell, if the pressure in a scuba tank will go up 5 lbs of pressure for every 1 degree of temperature change. So if going from 50 degrees to 100 degrees, it would only climb from 3000 to 3250 - and besides, just like paintball tanks, there are burst disks, so the tanks should not "blow up" no matter what you do - as long as they are good, inspected, hydro'd tanks.