If you really want to improve selectivity on the 455kHz IF, you have to go with a Mechanical filter.
Follow ebay and sometime you find cheap ones.
**broken link removed**
You have to search for a filter with bandwidth between 6kHz and 9kHz (looks like you are looking for AM bands).
Depending on the existing IF amplifier, using mechanical filters is possible to need an extra amplifier to compensate for their insertion loss.
Those amplifiers helps also for their impedance matching, which is totally different than that of ceramic filters (or crystal filters).