Get a copy of MMPDS, or even an old copy of MIL-HDBK-5, either has the data you want.
For either alloy, the ultimate strength for samples exposed to 500F will be 50% or less of the room temp. value for 1/2 hour exposure, and something less than 20% for 10,000 hours exposure, when tested at RT. Strength at when tested at the exposure temperature will be significantly less.
You need to rethink the clamping of your mold. Something like a stack of Belleville washers on a decent steel alloy tie bar might make more sense.