The selectivity of the filters is chosen to suit the bandwidth of the signal that you are receiving. This receiver project appears to be designed for radio direction finding, being an SSB receiver set up to receive LSB on 80 meters. For SSB, it is typical to choose filters for a cascaded response providing about 2.4 to 3 KHz overall bandwidth. When researching alternate filters, I found that the AM filters available are often quite wide, so I chose the narrowest model, the 739-455-HL-C. This part has a spec BW of 4 KHz at -3dB, so when two of these are cascaded, the net response is likely to be close to 3 KHz. Of course, you can choose any bandwidth you want from the Murata lineup, but I recommend going with something around 3.5 to 4.5 Khz per filter, or even a bit narrower is OK.
The impedance of the filter will affect the efficiency of the match to the 440 IC, but this is not a highly critical match. Note that there is a 3.3 Kohm resistor right beside each filter. If you change this value to be the same as the filter impedance, that is about all you need to do.