David, I recommend using B+W MRC ND filters; they are available in the filter thread sizes you need (77 and 72 mm, if I remember correctly for those lenses).
The Singh-Ray has a special ND filter that allows you to adjust the "density" (i.e. the amount of light blocked) by turning a ring. Very handy in some cases, but probably not what you need.
For polarizers I also recommend B+W MRC circular polarizers.