Hi seaking7641,
Maybe your best help is "only" properly designed guarding & shielding/signal cables?
Check pls other topics here at Edaboard as "guarding" for ECG, low level signal aquisitions I remember for some...
You can find good application book by Keithly an order it free per online too...
Self active 50Hz filter are bulky and you need more stages maybe...
K.