855 no boost. No power

I think you're expecting far too much from the engine.
I've pulled loads between 100k and 135k with 350 Cummins engines and to expect a cruise speed of 65 mph plus is a pipe dream.

Properly geared to pull those weights you would have a flat out top speed of 65 mph at 2100 rpm or higher to be a steady puller.
A 4:11 gear with a direct drive trans would get you there, around 60/62 mph @ 2000 rpm on 24.5 rubber which was a common set up back in the day for pulling heavy, over 100k.

As it stands, with the 18 spd you have 2 top gears, (OD gears), that will never be used efficiently or effectively under a load and are basically useless unless you're light or M/T.

If you think a Formula 350 is going to pull anything like a C-15, give your head a good long shake.
You might as well melt that Cummins down and make paper weights out of it, before it melts itself down.

Try this, run your truck in 7th Hi under a big load, (which is direct gear or 1:1), at high rpm's and see if your mpg increases, 1800 to 2000 rpm, 55 to 62 mph.

 

So if I replace this engine for 350 formula I will have truck compatible with my cat c15 to pull any heavy load?

 

No.
You will have a 350 hp mechanical Cummins engine that is nowhere near as powerful as an electronic C15 Cat and you need to drive it differently and lower your expectations.
To go from 500 hp @ 1850 ft/lbs of torque to 350 hp @ 1100 ft/lbs of torque is a huge difference, and no matter what you do to that Cummins it will never do what what the Cat will.

Sure, you could jack that Cummins up to 500 hp, but the engine wouldn;t last very long if you don't know how to manage the power.
I know this for a fact, I blew up a hot Cummins engine and paid the price.

What's faster and more powerful, a a Dodge Hellcat Challenger or a souped up Dodge mini van?

 

Thanks is all I need to know we learned at lot with other people experience and this BC is something new I will move over and look for other cat
Thanks