>"how does a head bolt "stretch"?"<
Don't know about the twin-turbo Supras, but most/many single turbo and non-turbo Supras have lost their head gaskets. It's called BHG (blown head gasket), and was caused by the factory undertorquing the head bolts. Toyota wouldn't do anything about it, unless it blew early.
The fix is to make sure the block and head surfaces are nice and flat, and increase the torque by ~15+ ft. lb. Better is to use ARP head studs. When you get to the 500+ HP levels, you need something better--fatter is stiffer.
BTW, the stock bolts just stretch elastically under high enough boost, just a few thousanths--after using up all the preload (torque-induced stretch). That's enough to let the fire out and wash your windshield.