[Highboy34]

.......an "It worked for me' suggestion. Years ago I helped restore a 39 LZ coupe. The owner had a set of NOS upper radiator hoses he wanted to use. They were hard as a rock, felt like they could be brittle, but he insisted. So I suspended them in a large pail of water on the stove. I heated the water very slowly to about 160 degrees and lifted them out. They were soft and pliable, not flexible, but they responded to hand pressure fine. Original thermostats were installed into them and they're still on the car without any problems to this day, some twenty years later.