One problem with ferrites is that the permeability (and thus inductance) depends on amplitude, frequency and temperature You mustn't let a permanent magnet get near it either, magnetised ferrites will change behaviour (and probably drift in permeability for ages afterwards). Amplitude isn't an issue for a AM receiver though
Either allow a lot of slack (ie have a wider-range tuning capacitor) or expect to wind a trial winding and measure the tank circuit (grid-dip oscillator is the traditional tool). Its traditional to make the coil have an excess of windings and to fix it towards one end of the rod, since moving it will allow fine-tuning to that particular rod. Application of molten wax to hold it once the placing is determined.