Are you certain they are leaking at the threads, rather than through the switch?
There is a thin fiber washer that goes under each switch, that also doubles as an adjustment washer for the overdrive switches. There is a procedure in the workshop manual for figuring out how many of them you need on each switch. TRF carries them under P/N 502146 (which is also in the manual). Looks like the Moss P/N is 324-145