Not sure how tall you are, im 5'10", i size up, twice, in one pieces andthat seems to help. I havent found anything in terms of suits for the longed torso, its sort of been hit or miss.
I'm not as tall as you... I'm 5'7 with a long torso. One piece suits just don't work on me. I wear TYR two piece suits to swim laps. There are a few other women at my local pool who do the same thing. Mine have always stayed in place without me having to even touch them for the whole swim.