To be honest, on a build like that, why do you care about mileage? I sure wouldn't want to be driving that on the street anyway. I hope you are planning on porting the 317 heads, because with a 238/240 cam, you are going to be running out of breathing room with those heads, especially on a 406. The last thing you want to do is have such a huge cam that you have no bottom-end, and then have no top-end flow because of your heads. That leaves you with a very narrow mid-range pull, and, to be honest, a dog of an engine compared to what it could be. AI does some really nice CNC porting on the 317s/243s that would be more appropriate for that cam.
I'm planning on an LQ4 with CNC'd 243 heads from AI (243's with a 64cc chamber will get me around 10:1 compression) with a nice mild bump stick. The AI 243s will gain probably about 20-30whp over stock 317s themselves, plus the bumpstick should add around another 30-40whp when combined with the heads. The nice thing about AIs program is that they focus more on port speed than flow, so this thing will still give you a kick in the pants down low, and pull hard up top with the additional airflow they provide. 400+whp and a killer bottom end? I say, why spin it to the moon if you don't have to?