Stuck thermostat?

I have a 1980 Olds Cutlass 260V8 rear wheel drive. Wife just got home after a long drive in 90 degree heat and noticed it was leaking coolant from the overflow tank when she parked it. I opened the hood and the overflow reservoir was all the way to the top! It was at the proper level the other day.

After waiting 15 minutes, I restarted it and the reservoir tank completely emptied into the radiator but I noticed the upper radiator hose was being sucked/collapsed closed unitl I shut her off. The hose was not collapsed when I originally opened the hood and noticed the coolant was all the way to the top of the reservoir. Any ideas besides the thermostat stuck closed? THANKS!