Half a year or so from now, dig a hole into and check for too much muckiness or water-rot stench. If there is a problem, you might be able to auger in from the slope to insert perforated drain pipes.
When you do plant, dig all the way to the bottom of that trench to fill with non-organic matter, so that the plant's root ball protrudes an inch or more above the surounding soil, so that the root-crown of the plant will not end up in a deep basin, or buried--- sandy loam, perlite, charcoal (former organic matter, won't decompose for millenia), rock dusts.