1970 Cadillac Eldorado coolant overflow

I've got a 1970 Cadillac Eldorado, about 58K miles.
When driving it, it's fine. Sometimes after a ride I stop the car and the plastic coolant tank (outflow hose) will overflow and spill some coolant onto the street.
This never happens WHILE the car is in motion, and I can always start it up again and drive it if I want to right away.
I heard a gurgling noise when I had the hood open once when I tried to diagnose it. Came from the middle-to-back of the engine compartment on the left side, then through the tube and then .. no surpise ... coolant surges into and overflows the tank. Anyone have any ideas why this happens? I have not added or removed any coolant recently. And I've not seen any coolant on the street before this either.
Is this simply a thermostat problem? Do I need to replace it? Thanks in advance for any ideas on this.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Having had one of those BEASTS, I'd suggest you take it in and get a power flush. There is probably scale all over the cooling system. Is that the original block or has it been rebuilt? You can check the thermostat too. Also the radiator cap. Do a pressure test. That engine is very sensitive to having the right pressure. If there is any leak it will boil. Mine was a royal pain in the ass after the car was 2 years old.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.