Recommended pin to control SSR for spindle?

I’ve been looking at using M42 to switch a solid state relay from inside the code to activate and (especially) deactivate either my spindle or my air assist compressor once a job is over.

Can anyone recommend a pin to use for this purpose that won’t cause any trouble with the other operations of the RAMPS set? My SSR is a 40 amp with 3v to 32v input.