[mongocanfly]

I'd say if you expect 20mpgs from any 6.0, your gonna be disappointed...
Like real bad..
My C30 has a similar setup, but is 2wd
I have a LQ9 backed by a nv4500, 4.10 gears, and 31 in tires..
If, and only if , I can keep my foot out of it, it gets mid 16s..
Put a trailer behind it and you can forget about mpgs.
On the dyno , the 6.0 turned 344hp and 399tq..
but it likes rpm.. based on my driving , the 6.0 sweet spot is around 2000rpm..
Only mod to this engine is headers, which I had to do to get around the clutch linkage,
Should have done hyd clutch
Manifolds will flow as good as headers I'm told

On my IH build I have a 5.3, with a stage 2 btr cam..backed by a nv4500,
I have 4.56 gears and 255/85-16 bfgs (34in tall)
Mods on this engine are btr stage 2, Holley intake and fuel rails, headers
By my calculations, the 65mph rpms should be really close between the 2 trucks..
IH isnt on road yet , so I dont know what kinda mpgs or hp numbers its gonna get

Its gonna cost big time to connect a LS to a nv4500.. unless you can find all the factory stuff..
I couldnt, had to go with all advanced adapter on both of these trucks

C30 2022rpm at 65mph
IH ..2055 at 65mph

If you want the classic look , that's gonna cost as well..
I did it on the IH, feel free to check the engine out,
And dayj1 did a superb job on his ,( I copied most of what he did) on his build called "another way to skin a cat"