ISX 870 Rebuild/Replace

[Chamberpains]

Im not gonna assume the crank and block are junk. Have you had it looked at? Is an inframe still possible? Over sized bearings?
If your looking at only 3.5 years of service and 200,000 service miles, what do you plan on doing with the truck after you can no longer use it? It will be 15 years old. So is it off to the bone yard because its just whooped? Or will have no resale value? If thats the case you may want to do a used engine search with the ecm with it. Take all the knowledge from here and have the pan, valve and front cover pulled. Inspect everything maybe throw in new main bearings, front and rear main seals, run the over head and check the timing with cam wedges. Pull an oil sample and have it checked. Hook a battery up to it and check the ecm for major abuse or neglect.
A quick ebay search shows a couple of these for $8500 there are other sources too. You might have 2-3 days into a full freshen up and maybe another $1000 in parts. But if everything checks out you should easily have a 200,000 miles engine.

My cummins long block was $19,700.
Do the math for your situation and use the sound knowledge you learned from here. That should guide you in your choices.