I agree, it's very important to let it dry for 4-7 days or what ever time is needed to dry out, and then bring in sandy soil, spread it about 4" deep over your muddy clay rich area. Don't let the delivery trucks anywhere near this clay, off load as close as possible and use your tractor bucket to help spread it. Compact your base and slope away if possible. Discontinue any gravel placement, because over time and with repeat rainy seasons, your high density gravel, eventually just sinks into the clay and will lose its effectiveness. The overlying sandy soil is of lighter density, and stays on top of the clay. Additionally, the sandy soil maintains good permeability, and water soaks through the layer quickly. As soon as you have compacted and graded your sandy soil, begin the process of planting field grasses to increase ability to support vehicles.