it really depends on the pants, if they are low risers you can typically get away with a size or even 2 smaller since we aren't as endowed in the rear. I would guess that a 42 waist would work out to a 18-20 in womans for a regular cut, and a 16-18 for a low rise cut, but as always YMMV.
the 30 length will usually be fine, any longer and you would want to get the same size in tall.