There is a big difference in hatches between the South Fork and the Henry’s Fork in March. On the Henry’s Fork you’ll get good hatches of Blue Wing Olives and Midges while the South Fork you won’t have Blue Wings until mid April. So if you are looking for a bigger dry to fish go to the Henry’s Fork.
Also consider fishing a sparse black or brown leech with a sink tip or sinking line. Cast slightly down stream and fish the leech like a soft hackle. The trout will not hit the leech or steamer hard but instead just tighten the line up and basically hook themselves. Remember to mend and slow the fly down.
For nymphs a Bennett’s Rubber Leg in any of his color versions are good patterns to use all Spring.