[DeTomaso] water pump , thermostat question

[boyd casey]

Hi. I was just driving  my Pantera getting on it a little bit when all of a
sudden steam started to enter the drivers compartment. At first I thought
one of the hoses to the heater core went (I don't have ball cock valves)
but it wasn't as much steam as I would expect if the whole system went .
The thermostat had not been reading hot  but quickly went up over 220. I
turned the car off and started checking things out. The first thing I
noticed was that the radiator was ice cold. , then I checked the pressure
bottle ,It was around 180 (I was lucky I had my laser thermometer with me.
 I also noticed that the right side head (passenger side was hotter then
the drivers side). Will a bad thermostat keep all the coolant trapped in
the motor? I had just left my house and had only been driving for around 10
minutes when this happened so the car was cold when I started. I let the
car cool off enough to get it home (I was only around 5 blocks away) and
then after letting it cool off for around 11/2 hours I went out and pulled
the firewall cover and hatch. the block is still fairly warm to the touch
(hot) and I thought I could hear a hissing sound coming from the thermostat
housing ( I have S.S. water pipes)  I checked the pressure tank and it was
almost empty. I took the car for a long drive on Saturday out to a car
barbecue with Bob Levitt and we were cruising on the highway at 80 + and
 the car ran cool in spite of the hot weather , so I know the car had
coolant in it. so before I start tearing things apart I would be interested
in the opinions of those more knowledgeable then me. Could this be just a
bad thermostat? If so what is the correct thermostat that I might be able
to source at a local auto parts store?  Any special info as far as a
replacement water pump?
Thanks in advance for your help.
Boyd