Is it possible to filter out noise to a pulse counter when there is no twisting or shielding on a long conductor?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Reorganised component layout.

I would be inclined to move R2 to form a low-pass filter with C1. The circuit isn't great, however, as you're having to compromise the filter with the pull-up (R1) which, with R3 and R4, loads the filter.







simulate this circuit

Figure 2. Opto-isolated circuit.

The circuit of Figure 2 has the advantage that you can optimise the LED / pulse switch circuit for noise immunity separately from the logic circuit (which still needs a tidy-up). R2 / D1 form a low-impedance circuit and it is very unlikely that your noise will have enough energy to light the LED. Q1 does the pull-down for the logic.

Replace 400' of cable or try an opto-isolator? I know where I'd start.

If you don't have an opto-isolator to hand you might be able to make one: Is it possible to use an optointerrupter as a optoisolator?