GMC did offer the Detroit Diesels factory with big trucks, I believe 450 size and larger, starting with a 3-71.
That Cannonball appears to be factory (and should be), it is a little unusual in that it has a 4-71, as many of them had 6-71's.

I was all set to buy a Cannonball with a 6-71 a few years ago, but was quickly talked out of it by an older friend and fellow GM truck and Detroit fan. He too likes the looks of them, but has some experience operating and working on them...

Thanks,
Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-