you are probably trolling but here goes:
The purpose of high mileage is aerobic development, At some point due to the law of diminishing returns such high mileage would not make a huge difference, so the difference in performance between 250 miles a week and 350 is minimal. Even paul tergat doesnt go over 210 mpw in his peak mileage.
seeing as you had already put in 120 miles weeks, you can increase that to 150, try holding that for about a month or two and see how you feel. If everything is going fine you can increase to 180. but you would want to stop there.
The next step would be to make your long run gradually faster, this run should be ideally between 20 and 30 miles. The way to make your long run faster would be to run the last 20 minutes at a faster pace, then after 2 or 3 weeks it should be the last 25 minutes, and so on.
When you reach 180 mpw your mileage should be run at abou 8/8:30 pace. But you can VERY GRADUALLY up the pace over the course of MANY months until you reach an average of 6 minute pace for all your (except for easy/recovery runs).
so basically at a certain point increasing mileage would not help, and higher intensity would be a good idea.