12.7 fuel milage help

Have 1.5 on my 12.7 Detroit rebuilt it full rebuilt turbo injectors and all except manifold was getting 7.8 whOle year didn't check in a while noticed more fuel stops checked is 5mpg I see the manifold might be leaking few spots and muffler sounds a bit clogged... you that be enough to throw it off? Did an overhead after like 50k you guys think that's all I need done? Engine sound and works without a struggle too

 

I ran 70 75 getting 7.5 to 8 goING east to west the muffler sound clogged and rattles and the manifold apears to have several leaks think I'm going to change and see what it does was just planning to get input so I can do it all at once thanks guys

 

Truck is like my baby hate to see her hurting makes me uncomfortable