I used a crossmember form a 78 GMC 6500 in my 48 Chevy 6400.
http://rides.webshots.com/photo/2236622090080251109tywgjR

Needed to make channel pieces to bolt inside the frame and narrow the corssmember some.

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-