The 450 motors with K-jet were the worst idling of all the V8s. You will probably want at least 8-900 rpm in neutral to get the best situation. The speed is adjusted by the large screw at the front center of the motor. Unfortunately proper setting requires adjusting the fuel mixture at the same time and that requires a gas analyzer as there is no electronic control on that motor.
one other expensive problem that takes about this amount of time to occur is the failure of the thermostaitcally controlled addition air valve. As it fails it requires higher temps to close fully. When this area reaches the normal range of the operating temp of the motor then the idle speed changes by how hot it is running at the moment.
Bosch Master, ASE Master, L1
33 years MB technician