Cummins 400 cuts out under heavy throttle

If it's a multiple cylinder misfire or across all cylinders, I would investigate a fuel issue.
If it's a single cylinder rapid misfire mid to upper rpm range pulling hard, I would look at the cam.
If it were my truck and I wasn't a truck mechanic for 33 years, I would also do the simplest things first.
Failing injector lobes can still provide enough travel to fuel the cylinder at idle and low load without a miss.
The engine swap suggestion was simply that, a suggestion for an option to consider.
Bigcam 4"s come from factory with LPF bores.
Cummins are great engines and Cats are too. If I had one of each, I would pick the low cost option to get rolling.
Not trying to over magnify the problem at hand, but offer some insight to help the guy cut to the chase.
Merry Christmas!

 

I caught the post about the BC 4 just after I originally posted. I knew that they went to the LPF's - but wasn't sure whether it was the 3's or 4's.

At that point, I would change everything in the fuel supply side. If you have a water separator, drain it and change the element. Then buy a tank of fuel, cut it with 10 -15 gallons of K1 kerosene and put a good fuel conditioner in it ( Power Service, etc....)

If you ever get the truck down to where it's on fumes, you would be amazed at some of the critters than can lurk at the bottom of a fuel tank as well. In the older trucks, I used to shut down the crossover, run it out of fuel on the feed side, clean out the bottom of the tank (remove the valves, and clean everything, put it all back and then open the crossover and you're good to go... )

Good luck and Merry Christmas.