Thermal properties of aluminium alloy 6063
American Society for Metals has many publications. Temperatures below ambient are no problem for any aluminum alloys. They are commonly used for natural gas  separation liquefaction exchangers and piping down to −100 °C (guess at the temp.).
In the T6 condition (hardened) room temperature yield is 31000 psi; at 300 °F the short term yield is 20000 psi; 6500 psi at 400 °F, and falling fast as the temperature raises.
ASME Boiler Code will have allowable stresses which will be lower than these values. I looked in ASM Materials Handbook vol 1, 1961, it is probably not online. If this is long term, you will want creep or stress/rupture data which is not commonly available for aluminum; try in the aircraft industry.