I am looking at swapping my unreliable 351 EFI out of my 150. I have been told that in such a small truck ( regular cab long box 2x4 ) my fuel economy will remain relitivly the same. the questions i have are, if this is true, will i need a bigger rear axle and will a 5 speed ( M5OD ) stand up to the power? I know that the f-350's had dana 70's or 60's. Any feed back will help