355 to 308 ratio?

[Waterloo]

I'm running the 3:42 with the 600hp 18 Spd, and can tell you your mileage will not differ by much at all, if any. I get my best mileage running in high 7 at right around 60 +/- 2 mph. The rpm is right around 1500 - 1600. I'm pulling a van, mid roof, so my fuel mileage is usually around 6.5 mpg, my truck is a bit of a pig, a heavy pig. I would stick with what you have and keep that money in your pocket. Like Rawze and others have said, you will just beat the motor to death... And having been through two in frames in my ProStar, you don't want to go there.