Could it be you have an air lock in the block? It is suprising to me that you can hear it boil in the block. I have a 216 in my 37, But for the life of me I can't remember if I have a thermostat in it. I know I run about 180 to 190 on a 95 degree day. I guess the things I could think of is:
Air lock
The impeller is corroded and not circulating
Pulley slipping on the shaft
Bad New thermostat
Collapsed hose (I know you said new)
Sludge in the block
Wrong head gasket? I have never heard of that one though on a 216.
I think if I was going to take the route of sludge, I would knock out the 2 side freeze plugs, stick a hose in it and keep flushing. I got alot of stuff out of mine doing that. Beats taking the pump off etc...
Good luck, I would like to hear what the final problem is.
Brent
