Well I had the same problem once. It's a typical frequency conversion: you want to convert a signal from an high frequency to a much lower frequency. The problem is that both your input frequency and LO frequency are very close (450MHz and 430 MHz) so it's very hard to eliminate the LO harmonics from the input because it would require a filter with a very high Q factor (5th order filter or more).
That's the reason why in most cases these kind of conversions are done in 2 or more steps. For example, using two conversions with same LO frequency:
1) Convert from 450 to 235 MHz using a 215 MHz LO
2) Convert from 235 to 20 MHz using a 215 MHz LO
As you can see, in this case, the LO frequency s well separated from both the input and output frequencies, making it easier to filter. Using the same LO frequency in both cases simplifies the circuit.
Thanks for fcfusion's suggestion.It is really a good idea.
You probably don't want to completely change your circuit right? If that's the case I can't think of way of solving that problem, I'm sorry. Perhaps you can still use band-pass filters but don't expect much improvement. Using filters with ressonance can give you a nice filtering performance and that's how I solved in my case. However, that requires precise L and C components which are probably not available on the market. This means that you may not be able to implement the circuit in real life or, at least, it won't work as well.
Wish I could help more.