i have interesting situation

[Budfreak]

me and my dad are putting together a 1977 nova RS with a olds 455 motor. when we start the engine and run it for any length of time, as soon as you kill the motor antifreeze begins shooting out of the overflow and the radiator is full of suds bubbles! we don't think there's a thermostat in it. theres no oil/antifreeze mixing and we checked the intake with spray and found no leaks. we are completely stumped!!!! also, we can't get the idle below 1500 rpm and the idle screw is all the way out. the engine won't even die when we tighten the mixture screws all the way in, it actually speeds up! we found every vacuum leak we can and plugged them all off. we found 4 ports that were open and we plugged them but it only brought it down 300 rpm from the previous idle of 1800rpm. HELPPPPPPPP!!!!