I'm not a big fan of the approach you've taken on the 1 ton demoted to 1/2 ton thing, but that's just my opinion.
However elongating the U-bolt holes 1/4" with a file should be just fine, provided you use some good hardened washers and heavy nuts like they'll sell you when you buy your new U-bolts.

If you've used a torch then depending on what was done I'd probably look for another axle.

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
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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-