It's not frame length that determines what size brake lines you need, it's the brake system components that are important. If you still have 3/4 ton brake system you need the right size lines for it.

Tube nuts are what go on the tube before you flare the end.
They look exactly like these, and you can see the different sizes available.
http://rides.webshots.com/photo/2824036070080251109kYsvFR

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-