Friggin need advice

[WS6]

If you want 25 mpg minimum and 500 rwhp you only have two options. Boost or nitrous. You're not going to get those numbers any other way. Now if you say you want 20 minimum and to stay NA, then you should be able to get 500 if not be very close. I would do like mentioned and look into the L92/L76 setup since you're a 6.0 you can easily do this.

CC is right, a stock 6.0 will not like 6800rpms. You can easily modify the engine to handle those rpms though.

Oh and don't waste going to a 383 when you can go 402 with just a crank change.