Same issue with a 1/2 ton... people either shim the box out from the frame and or some magical exhaust manifold.. or a different box alltogether.

If you're going to use a different box i'd look for something more modern than an f6.

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-