RabbitMQ.Client を使用すると、IModel.CreateBasicProperties() メソッドを使用して取得できる IBasicProperties を使用して配信モードを設定できます。
using (IConnection conn = factory.CreateConnection())
using (IModel channel = conn.CreateModel())
{
channel.ExchangeDeclare(exchange, ExchangeType.Direct, durable: true);
channel.QueueDeclare(queue, durable: true, exclusive: false, autoDelete: false, arguments: null);
channel.QueueBind(queue, exchange, routingKey, null);
var props = channel.CreateBasicProperties();
props.Persistent = true; // or props.DeliveryMode = 2;
channel.BasicPublish(exchange, routingKey, props, Encoding.Default.GetBytes(message));
}