Have a picture of the frame rail as they extend out the front?
I ask because the 1 ton and the 4100 1.5 ton have a lot of similarities and the frame at the front is one of the differences, otherwise the frames are nearly the same given the same 137" wheelbase.
Could be you have a 4100 frame and not a 3800 frame?
I'm unclear how this relates to the fit of the grill...

•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
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-