If you are doing this for a friend, figure out your labor costs per hour or per day and add any materials you will use and bill accordingly. If I were to plant those for a paying client and was going to guarantee them, we get around 3X the cost of the plant, installed. That's after figuring plant, material & labor costs and adding in profit.
Your best amendment for planting these trees is compost or humus. There is no need or reason to add peat moss, as it has no nutritional value, will lower the PH due to it's acidity, but will retain moisture. Just putting a layer of topsoil on the ground is like putting butter cream icing on a nasty cake. Once you get through the top layer, it's still nasty down below. Leylands are very shallow and fibrously rooted, so you may have to stake to keep them upright if there are open to winds. because they are an evergreen and do not go dormant in the winter, they will need to be watered on a regular basis so they do not dessicate during the dry, windy periods of winter. Another additive worth considering, especially at this time of the year in questionable soil is something with mycrorhizae, like Roots transplant Step 1, Espoma Bio-Tone Starter Plus or similar products. These products will encourage root growth, provide some natural fertilizers and many contain a water holding gel so the plant roots do not dry out between waterings.
Plant them high & they'll stay alive, plant them deep and you will put them to sleep. Your better off planting them around 10' apart and staggered if you can rather than in a straight line. A leyland hedge can and will reach 30'-40'in height relatively quickly. Leylands, when happy, tend to double in height every 3 years. Also, by planting only one type of plant, you risk catastrophic losses from pest or disease problems which are very prevalent with leylands. You may want to mix in some Green Giants for a difference in texture and color and plant a little tighter to achieve similar results.