RF Design Question: 40MHz to 10MHz sine wave?

Is the 40 MHz manufacturered digitally? Do you have access to the clock/driver oscillator?
Is everything analog? Can you add a 30 MHz signal and filter out the resulting 10 and 70 Mhz beats?
You can synthesize a 10 MHz yourself with either a 555 timer or a Statek chip.

Well, I could use a 30 MHz signal mixed with my 40 MHz reference, and filter out every product but the 10 MHz I want, as you said. Or, I could use a comparator to convert to digital, use a PLL to divide down to 10MHz, and use a LPF to go back to analog. Any clue on which is easier and/or cheaper? Or any premade box that would do it for me?

Are you trying to extract content around the 40 MHz. I figured you wanted the bandwidth the same but shifted to 10 MHz vs “simply” dividing the 40 MHz down. It was the “RF” in your question that made me think that.
If there is no RF content and you really only need the 10 MHz how about making your own 10 MHz oscillator- (very cheap) and gate it with a signal indicating the prescence of absence of the 40 MHz. As for a pre-made box, check out any of the triggered Wavetek boxes. There are many different designs I don’t know what kind of noise levels you’re working with.
Check the National Semiconductor Linear Applications book. That might get you very close.