Inserting digits

At the moment, the number starts with a $5$ . After inserting three digits it will still be like this unless we put a digit in front. Putting a $4$ there would make the number smaller but a $6$ helps. For the $4s$ , the places in front of $5$ and $8$ aren't good because replacing $5$ or $8$ by $4$ (you're not replacing them, but for the magnitude of the number only the left digits count, so you would first look at a $4$ and then a $5$ ) would make the number smaller. The best place is in front of the $2$ , which gives the result of $\boxed{658442419}$ .