Big block, small block, 6 cylinder, gas, diesel, who cares? Do what you want.
Anyhow, I don't see why it couldn't be done in a clean fashion. Guys have been stuffing GMC 302's in AD trucks for years. Sure there was some cutting and hacking, radiator moving, etc... That's part of hot rodding. The GMC 302 was long and heavy, not a whole lot different than the Cummins. Not to mention, the AD trucks came with short water pump 235's whereas the TF trucks had the long water pump version, which tells me they had a longer engine bay.
I know the short 235 is 30" from bellhousing to water pump. From research, the Cummins is 34 to the water pump, minus the fan.
The Cummins will have more to it than the venerable inliner. Such as, Turbo, intercooler, and all it's plumbing. So that may become an issue.
The Cummins will weigh a bit more than the old inline. But, if you've ever pulled a 235, 261, or 302 you'll notice they aren't exactly light.
Some frame boxing plates will help. Adding some crossmembers, will help even more.
A set back firewall may or may not be needed. I would be willing to guess a larger trans hump will be necessary.
Here's an AD truck with a Cummins.
https://www.youtube.com/watch?v=-5cKNhC--Y4