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You have two ways. 1) build two higher frequency oscillators, mix them together, and take the low side IFoutput. One oscillator at 432 to 510 MHz mixing with a fixed 400 oscillator, for instance. or 2) get themost hyperabrupt varactor you can find and get two or three different inductor values to cover the range, and use a PIN diode or fet to switch in the different inductor values.
You should be able to cover the range using some parallel hyperabrupt diodes (BB640 for example) without switching the inductances. I've played with such oscillator and it achieved frequency coverage from 77MHz to roughly 300MHz... (BF998 + BB640s were my friends)